[i am not ranking mega and base forms separately, other than beedrill almost every mega has reasonable effectiveness in base form to make dual mega a potential option and i dont know how to capture that in a ranking system.]
S, S-
as everyone above has said, melm's defensive and offensive presence esp vs fastest megas is basically mandatory to any team, and mew maybe isnt needed but theres rarely if ever a reason not to use it.
zapdos is a huge offensive threat if specific checks are gone as well as being the only defensive melm check in the tier. it has the choice between a pivot set to gain momentum for the team or a substitute set which many of its standard checks struggle with or even outright lose to. teambuilding wise, zapdos has excellent synergy with gyarados as collette described extensively here
https://www.smogon.com/forums/threads/lets-go-overused-discussion-resources.3667865/#post-8588321 [and similarly blastoise], it is mandatory on m-bee teams as well as an additional pivot, and also supports zam and aero very well due to their poor melmetal matchups. so it fits well on the majority of teams
i would also be okay with mew being in zapdos rank rather than melm rank
A+
to me there isnt enough of a gap between any of the top four megas to justify separating any of them in different ranks. i dont rly have anything substantive to add to others' descriptions above.
i do tend to agree with lily that zam and aero are a bit above gyara and bee, but imo not by a full rank.
A
almost every team right now should have a ground type, and these are the first two to consider. if you want a ground type that sets stealth rock, u usually want nidoqueen, which has enough bulk to set rocks in front of almost anything if needed, checks pivot zapdos, and is sufficiently threatening with eq toxic and super fang. it also has the option to run jolly to outspeed the crowded 80-81 base speed tier, particularly on zam teams to eg get a super fang on mega gyarados, or to run dragon tail to prevent setup (cloyster) and punish switches; and maybe some other possibilities like sludge bomb for substitute zapdos, cloy, and the poison chance in general.
if you are using something else (ie mew) to set stealth rock, then dugtrio-a generally has the most to offer with its speed tier, ability to check all zapdos variants, and its steel typing which most notably makes it the only ground with a favorable matchup with alakazam. the sub + toxic sets that are increasing in usage can be very hard to deal with as dug-a sets a substitute freely against any zapdos without drill peck, while sucker punch provides rare priority that is useful against beedrill and zam. fitting all three moves and dropping rock slide could maybe be possible, as substitute sets often dont have any need to damage zapdos directly anyway, but this would prevent dug-a from being an effective switch-in to zapdos that run substitute.
A-
~ alolan muk is the tier's best general special tank, and dark is an excellent offensive type so it can be hard to switch into. however as lily said it gets worn down by zam/gar that it wants to check, hates burn, and really is not a good switch-in to almost any mew variant despite its typing; and also is generally forced out by every earthquake user other than melmetal, so it has momentum issues. foul play (does more damage to grounds and aero), acid armor or maybe self destruct are potential ways to improve its worse matchups or momentum, but for now i dont think it can be justified a higher ranking.
~ dragonite can sometimes be a dangerous sweeper, but tends to have issues with being forced to lock into outrage. an eq immunity is generally very desirable in this tier, but it cannot switch into either nidoking or aerodactyl so this utility is inconsistent. running fire blast lets it do much more damage to melm but requires dropping roost or eq on the standard set. dragonite has a very different toxic stall set (fire blast, toxic, roost) which can also run dragon tail in the fourth slot to force a lot of damage, which i encourage people to try. i think dnite can potentially rise if its versatility is proven more but right now it is a bit predictable and it is never really sweeping. the 4x ice weakness is very undesirable since ice coverage is common anyway bc of zapdos.
~ gengar is a very dangerous offensive threat as everyone above has elaborated, but it offers nothing defensively and the adequacy of its speed tier is somewhat matchup dependent. gengar has absolutely proven itself as deserving a place on many if not most m-alakazam teams, i haven't seen it to be all that effective outside of zam teams tho and that is the main reason i dont have it ranked as high as lily eve and collette do.
~ venusaur is an interesting mega option, its ability to check pivot zapdos and be a decent check to grounds opens up teambuilding options that other megas do not have. it also can be somewhat effective in base form alongside a fast mega such as alakazam. still, mega evolving venusaur means losing both the sweeping and wallbreaking power of any of the top four megas, while in base form it has poor physical bulk and no longer checks grounds. venusaur is a bit underexplored especially in teambuilding, idk i wasnt rly sure where to put it but toward the bottom of A- felt right for now.
~ poliwrath also feels awkward to evaluate its rank, but against most teams it will be a defensive check to melmetal and at least one other pokemon, and it contributes reasonably consistently as a toxic spreader so i think A- is okay for it. (personally i rarely use it)
B+
~ sandslash and nidoking are the two grounds u are most likely to want to use if not using queen or dug-a. sandslash is generally the best option for mega beedrill teams, where not being weak to earthquake and a favorable aero matchup is very desirable, while unlike rhydon/golem sandslash is still a very reliable rocks setter allowing mew to run a fire type move (which it cannot really do if used to set stealth rock). nidoking can be a good option on mega alakazam teams, as it threatens major damage to zam's checks with earthquake and/or megahorn, and megahorn in particular is very hard to switch into since the tier's common resistances to it have poor nidoking matchups.
~ cloyster is an excellent wallbreaker for zam teams, but it often has to set up on rock slide user which means a 27% chance to be flinched and not accomplish anything, and it faces competition from gengar.
~ persian-a continues to have low usage but excellent performance in tournaments. it has an excellent speed tier and solid stab move in foul play, utility in taunt to deny rocks and/or recovery, and u-turn. it has a bad matchup with the three fastest megas, and as a taunt user it does often depend on 50-50s, but these are really its only deficiencies. at this point it has only really seen use on m-beedrill teams, but imo it has definitely shown greater effectiveness than anything ranked lower and its 'current' placement in B- is far too low.
~ snorlax is an alternative special tank to muk-a, that isnt weak to earthquake and therefore usually has fewer momentum issues, as well as getting stab on self destruct. however, it lacks muk-a's resistances, normal is rly not a desirable offensive typing in the metagame, and it is slower than melmetal. while body slam is annoying to switch into, lax tends to be very dependent on the paralysis chance to be effective.
B
~ mega blastoise is a bit underexplored, it has a number of advantages to gyarados in addition to the primary feature of being a special attacker and therefore doing more damage to melm and not caring that much about burn. one is its lack of weakness to u-turn, which is a way that m-gyarados can easily get worn down, and also eg megahorn from nidoking or rapidash. another is priority aqua jet, which can be hard to fit but does decent damage to the three fast megas and in particular means that the m-beedrill matchup is perfectly fine. running dark pulse allows m-blastoise to run hydro pump without having pp issues against mew, while hydro + aqua jet also kos zapdos after rocks so it cannot switch in; however this comes at the cost of toxic and dragon tail to deal with poliwrath and m-gyarados switchins. the zam matchup is much worse compared to gyara but overall the lack of switchins to hydro pump on teams lacking two specific mons enables m-blastoise to be both a powerful wallbreaker while also providing the defensive benefits of a water type, much like gyarados.
~ i have starmie ranked lower than others do, it seems like it has potential but i need to see more from it. i think e(eve)e addressed its issues, the damage on an unstatused mew isnt rly adequate, water typing is good esp on something that isnt concerned about zapdos but it doesnt rly hit that hard, and it has major 4mss. its recovery is hard to actually use without a screen or status move, but then it tends to be vulnerable to status itself. its ok on zam teams running dual stab and toxic, toxic prevents mew from checking it and therefore can allow it to force damage on a target like muk-a.
~ egg-a is a valuable eq resist for certain teams, tho it doesnt help at all with nidoking so its a bit matchup dependent. it has generally decent matchups and also dragon tail, but outside of teams that specifically want the ground resist u cant rly justify the teamslot. i can see it in B or B-.
~ as lily said, zard x is a terrifying breaker in the tier, with fire stab a wide offensive movepool and also dragon tail to punish switches; but it is very hard to build with since it not only means losing out on one of the fast megas but it rly doesnt have the defensive utility that either venusaur or gyarados/blastoise do. (it isnt a terrible zapdos switch-in, but it does not like taking toxic). dual mega is rly not an option for it and its synergy with most faster non-megas is poor, it def has 'potential' to be higher but as of now its teambuilding issues rly hold it back and id need to see an effective zard x team to rank it higher.
B-
~ i am the only person who uses electrode rn but i absolutely think it has a place in the B ranks. its speed tier is obv very useful enabling it to be a late game cleaner for teams that do not use one of the three fastest megas, and in particular its ability to outspeed and ohko m-beedrill after rocks is excellent for m-gyarados teams. it puts pressure on opposing ground types as its thunderbolt often lacks switchins if the opposing ground is removed; and against m-beedrill teams switchins to foul play are limited allowing electrode to punish a m-beedrill ko and potentially countersweep.
electrode can be a bit prediction dependent as a taunt user tho not as much as persian-a. it is not splashable and it also has to be careful about when it chooses to come into play as bringing out electrode at the 'wrong' time can lead to a loss of momentum or being forced into an unfavorable 50/50. still, it absolutely has a place in the metagame. also to note that trode can be very affected by 'random' meta trends, most obviously adamant vs jolly mega aerodactyl, also adamant vs impish on grounds @ foul play damage.
~ primeape is the most relevant fighting type rn bc of u-turn, and has been getting a fair amount of use on m-bee teams as a pivot that threatens major damage to melmetal with low kick. its nothing amazing but it does its job on those teams and is not extremely niche in the way the C ranks are.
~ rhydon is a strong wallbreaker but it tends to be very prediction dependent bc of its bad speed and worse defensive typing. it is also not a good stealth rock setter, which means using it on a m-beedrill team prevents running fire blast or will-o-wisp on mew. idk, if it has more use and effectiveness with sub + megahorn i can reevaluate if i have dropped it too far lol.
~ rapidash and kingler are sort of similar to me, they are excellent wallbreakers, they really dont switch in and out their job generally is to come in on st they heavily threaten (eg melm for rapidash, ground for king) and force heavy damage. they should not be used to switch into anything and therefore can be a bit hard to fit on teams, but every so often they each offer desirable attributes for a specific team.
C+ (not rly ordered within the ranks after this point)
~ jolteon i find a bit hard to fit on a team, u will generally either be weak to at least one of the three offensive megas, or struggle too much with switching into grounds. if u specifically want an electric type for late game, u are generally better off with electrode's speed despite its ~50% weaker thunderbolt. however, jolt's general speed and power if grounds/venu are removed cant be ignored. jolteon has interesting synergy with aqua jet m-blastoise (dual electric), which rly appreciates screens and also has 'ok' priority for the fast megas.
~ golem has some advantages vs rhydon due to removing itself from play with explosion, eg if wisp mew burns rhydon as it clicks megahorn then it can just roost stall, but if it burns golem as it clicks explosion then mew has taken that damage and can be forced out without a chance to roost. also, golem is a bit less prediction dependent in a sense bc explosion hits everything very hard (except melm or gar which are terrible switchins otherwise), while rhydon tends to need to hit targets with the correct move (tho it can also run sub). regardless, they both have the same central issue of a terrible defensive typing and therefore being bad rocks setters and u-turn switchins.
~ sandslash-a has decent priority for the fast megas (and ofc also zapdos and dnite), and its rare steel typing offers many benefits including letting it check zam. theres decent flexibility for the last two slots after eq + ice shard, swords dance + x scissor is prob the best set 'generally' but eg counter, toxic, ice punch are options. it really is never sweeping without significant paralysis support, but it has the best priority in the tier outside of pika-s while also providing various defensive utility (its sort of like a worse melmetal), and really not having a lot of bad matchups with the exception of poliwrath and some mew variants. slash-a shouldnt switch into anything, and sometimes this mon can feel like u just wish u were using one of the ground types instead. i could see this potentially rising with more usage and if people can find clear team comps for it.
~ clefable feels rlyy awkward rn, a ground type rocks setter tends to be much more desirable due to their ability to practically get free turns vs zapdos, and outside of rocks as i think eve said it has generally decent matchups but it rly lacks any clear role, or reason to put it on a team.
~ eevee-s only gets so many turns and tends to rly struggle with zapdos. as eve said occassionally it looks rly good tho. idk could potentially rise but its sooo inconsistent rn, this feels like where it belongs for the moment
~ raichu-a rly struggles against dug-a teams, but otherwise it has a very spammable electric stab due to its ability to outspeed and hit other grounds/venu hard with psychic, and having options to prevent zapdos/mew roost stalling in encore, calm mind, toxic. it takes more damage than m-zam from u-turns but encore can help with this somewhat (ie if zap/mew comes in, use a damaging move first and then either lock it into roost or it has to pivot out without recovering). its rly inconsistent bc of its dug-a issue so ranking it next to eevee-s feels appropriate
C
~ i have only seen eve use mega pidgeot, its a mon that def has potential with heat wave toxic u-turn roost. (its movepool is tiny so its pretty limited to that, hyper beam is an option for a nuke but cant rly be used against teams with a setup sweeper, air slash can be used for flinch chance esp alongside toxic but otherwise its only a lil stronger than heat wave and coverage wise basically only hits poliwrath.) i think the biggest issue for mpidg is that almost the same set can be run by mew, mpidg having better speed, a slightly stronger fire move (~1.1x damage compared to mew's fblast) and a usually better defensive typing esp on roost turns, while mew has better bulk and is not weak to rocks which is obv important for a toxic stall pivot. personally i use toxic stall mew fairly frequently, but i have found it preferable to use the mew slot rather than the mega slot for that. still, mpidg has easily the best dug-a matchup of any mon in the tier, and def feels like it has enough to be viable tho its effectiveness still needs to be proven in practice
~ victreebel has several interesting features but it can be hard to justify over venusaur, which gets leech seed 'instead of' sd + sucker punch and even in base form is reasonably effective while also providing the flexibility of dual mega. the speed tier difference is also v significant. victreebel has most potential to outperform venusaur with paralysis support, which venu doesnt care about that much due to leech seed, as well as it conflicting with sludge bomb poison, but para support is hard to provide with any consistency in the current meta.
~ pika-s has been getting more usage, i still rly dislike it but it def has more relevance than everything in C-. zippy zap does ~1.5x the damage of sandslash-a's ice shard (which is slightly stronger than (modest) m-blastoise's aqua jet) to the three fastest megas, but its offensive presence is mediocre otherwise and it will rly struggle making progress against at least one of mew, zap, melm, venu, depending on its last two moveslots; and obv it has no bulk. 50% more damage is obv a lot but if u want a priority user to revenge the fast megas, i still think sandslash-a should 'generally' get strong consideration over pika-s since it is much more effective otherwise (its eq is reasonably strong even without stab, and having x-scissor for mew and gyara/mie), and also since it checks m-zam defensively anyway if zam is not in ice shard range. (or m-blastoise if u are ok with a priority user that takes up the mega slot)
~ chansey has been rly declining since mgar ban, i still think sing + teleport is its best set, by a lot, but it is obv inconsistent; and toxic sets cant do anything to gengar other than teleport out while twave leaves u walled by zapdos. (u could try toxic + fire move instead of stoss if u want but . yeah lol. the damage on melm is still pretty bad, and the damage on nidos is worse.) so please use sing if u are going to use chansey. u can beat zam 1v1 just with stoss anyway so u dont have to worry too much about activating sleep clause.
~ hitmonlee/machamp have ~interesting features, but it often feels like youd just rather have a ground type instead. one thing they can do much better than grounds is take on standard scald rocks mew, assuming they run facade (toxic + facade or bulk up + facade). machamp also checks (roost) m-aero, which grounds other than sandslash-k and to a lesser extent goldon do not do, while hitmonlee has a good speed tier. against teams without scald mew, hitmonlee is mostly outclassed by nidoking, while machamp is mostly worse than sandslash-k; the electric immunity is obv extremely valuable and the fighting types offer very little to make up for that. (eg while low kick / hjk do significantly more damage to melm than stab eq, eq generally does more than enough damage to melm to be favorable anyway. the greater damage does however make fighting types much more tolerant of burn, as their stab moves still do >30% to melm)
C-
~ dodrio's thrash is very strong esp at +2, while having jk for melm, but it is so frail and as usual normal is rly not a good attacking type. imo its not even clear that this is better than tauros, which doesnt have to lock in to dedge and therefore can toxic mew/zapdos to prevent roost stalling while also being able to hit melm with eq. anyway, i havent seen dodrio used in a rly long time.
~ pinsir basically requires dual mega, usually with aero, as its effectiveness depends on its ability to either raise its speed tier or keep its good defensive typing in base form. bulk up is still maybe a bit undertested as it can set up on quite a few things but regardless this mon is always going to be niche due to the extreme teambuilding constraints it presents.
~ dugtrio-k's niche (as explained well in collette's nomination) is to either hit 10% harder with adamant or reach a higher speed tier than dug-a can. however esp with the development of dug-a's substitute set(s), which dug-k cannot run as zap's u turn easily breaks the sub, i dont think theres a good reason to consider using this anymore. rly prob should be unranked
~ i put the three fringe special waters together, idk i think they are all a bit hard to justify but they have niches on paper. oma has no coverage for waters but it hits harder than cloy so it isnt truly dependent on shell smash, however unboosted it is just worse than eg vaporeon so it is sort of a weird middle ground. vaporeon prob could rise a bit higher, and it is a water type that can actually run scald if desired since it has shadow ball for mew, and a burned zapdos rly cant stall it either while also not ohkoing due to vaps excellent special bulk. however vap may be kind of hard to justify over eg kingler, which has a much stronger stab against anything other than melm. tentacruel has some interesting features, it is a poison type that outspeeds grounds other than dug-a and hits them hard with its water stab, when healthy it can force out standard zapdos with mirror coat, and counters scald mew. its dual stab has decent neutral coverage but tenta can be very prediction dependent, obv with coat but also in general hitting specific targets with toxic or the correct stab move. also its j very weak, eg hydro pump 3hkos melm. ive gone back and forth on whether it should be ranked, idk.
~ ninetales-a has a million obstacles, its outsped by a lot and also walled by melm, but its prob the tier's best np user besides mew, and it can set up pretty freely in front of muk-a and heavily threatens zapdos. its very questionable lol, if u want to use this consider playing uu instead.