Drops
(Ground) A--> D
Mon is trash, Mega Steelix does it's job of dealing with Bulu, and it doesn't offer much outside of that.
(Electric) C--> D
Electric doesn't need webs at all, and it's special attack is pretty lackluster to try and compete with Electric's other special attackers.
(Electric) B--> C
Manectric is very lackluster on current Electric teams, with plenty of other options fighting for it's spot, I think this mon fails to make meaningful impacts in most matchups, as anything Manectric does, another mon could also handle, unlike Thundurus, the only other B mon to compare it to, who adds to multiple matchups in a meaningful way.
Rises
(Dragon) A--> S
Given how important stealth rocks are, along with it's ground typing to help vs rock steel and poison types while preventing volt switches, Garchomp is hugely important to dragon teams and IMO is irreplaceable.
(Fighting) Unranked--> C
Dual screens taunt lead allows Fighting to set up otherwise unreliable sets like Double Dance Terrakion, or Dragon Dance Zmove Kommo, or just otherwise attempt to give Fighting some added defense to make up for their lack of defensive options, which is more valuable than Toxicroak IMO
(Electric) D--> C
I think specs or LO on this mon is pretty bad, but I think there's some potential in Vinkou with either Tbolt, Shadow Ball, or HP Ice, helping in matchups like Dark, Psychic, Electric, Grass and Fairy.
(Psychic) A--> S
Dark and Bug neutrality, able to break holes in almost any team and get around would be psychic problems like Muk, Tyranitar, even able to switch into them in a pinch, unlike Victini. Knock Off/Shadow Sneak to get around Ghosts, Gallade-M is uncontested as Psychic's best answer to it's weaknesses and should be one of the first mons on any psychic team that's not sacking matchups.
(Dark) A--> S
Hazard removal, Mega Lopunny answer, and Fighting switchin for non-mega Sableye teams, imo this is the most necessary mon on Dark teams to not autolose to fighting moves/hazards and should be S rank accordingly.
Mon is trash, Mega Steelix does it's job of dealing with Bulu, and it doesn't offer much outside of that.
Electric doesn't need webs at all, and it's special attack is pretty lackluster to try and compete with Electric's other special attackers.
Manectric is very lackluster on current Electric teams, with plenty of other options fighting for it's spot, I think this mon fails to make meaningful impacts in most matchups, as anything Manectric does, another mon could also handle, unlike Thundurus, the only other B mon to compare it to, who adds to multiple matchups in a meaningful way.
Rises
Given how important stealth rocks are, along with it's ground typing to help vs rock steel and poison types while preventing volt switches, Garchomp is hugely important to dragon teams and IMO is irreplaceable.
Dual screens taunt lead allows Fighting to set up otherwise unreliable sets like Double Dance Terrakion, or Dragon Dance Zmove Kommo, or just otherwise attempt to give Fighting some added defense to make up for their lack of defensive options, which is more valuable than Toxicroak IMO
I think specs or LO on this mon is pretty bad, but I think there's some potential in Vinkou with either Tbolt, Shadow Ball, or HP Ice, helping in matchups like Dark, Psychic, Electric, Grass and Fairy.
Dark and Bug neutrality, able to break holes in almost any team and get around would be psychic problems like Muk, Tyranitar, even able to switch into them in a pinch, unlike Victini. Knock Off/Shadow Sneak to get around Ghosts, Gallade-M is uncontested as Psychic's best answer to it's weaknesses and should be one of the first mons on any psychic team that's not sacking matchups.
Hazard removal, Mega Lopunny answer, and Fighting switchin for non-mega Sableye teams, imo this is the most necessary mon on Dark teams to not autolose to fighting moves/hazards and should be S rank accordingly.
A->B - Pursuit trapping Victini is more reliable than using Araquanid, and as a Sticky Web setter, it pales in comparison to Galvantula.
B->A - Forretress's access to Volt Switch along with especially powerful hazards in Spikes and Toxic Spikes makes it more competitive with Armaldo. Not having Knock Off is a huge blow and not being able to spin against Victini makes it slightly worse against Psychic. However, it is very consistent and should rise as a result.
S->A - Reflects the growing parity between Mega Sableye balance and Mega Tyranitar offense that Dark has seen recently. Mega Sableye is still a fantastic Pokemon, but a drop because the other S ranks are significantly more useful.
A->S - Mandibuzz is relevant on all Dark builds for its great typing, bulk, and access to Defog.
C->D - Worse than Weavile and Mega Sharpedo, it should be a lower tier.
B->C - The matchups Mega Latias helps with are more easily covered by others, specifically Kommo-o covering essentially every matchup that Stored Power helps with. Unlike Mega Altaria, which at least has the potential to cover Dark, Mega Latias struggles to do much unique.
C->A - Healing Wish is fantastic support, dropping Mega Latias is easier than ever, and there are some builds even Latios can be replaced by Latias on nowadays.
C->B - Toxic Spikes support covers the majority of Dragon's bad matchups, and its Poison STAB makes Fairy much easier to handle. It rises to be on the level of the other matchup fishing Pokemon.
S->A - With Rotom formes being able to Defog, Zapdos's role as a Defogger isn't as unique. Furthermore, there are other options for covering the Ground immunity in Air Balloon Magnet Pull users, Rotom formes again, and Thundurus for offensive teams. While Zapdos continues to be a great option for handling utility Tapu Bulu and non-Continental Crush sets, it is far from as mandatory as it once was.
A->S - Zeraora is what Electric has wanted for years: a fast, strong physical attacker. Its ability to run a wide range of sets between Flyinium Z, Choice Band, and Fightinium Z give it options to cover different threats depending on Electric's need. With good play, it beats many types that are traditionally hard for Electric including Fairy, Poison, Grass, and Normal.
A->B - With Zapdos and Alolan Raichu in A, it's hard to argue for Alolan Golem to stay. Electric teams can get away without Stealth Rock, and it faces competition from Magnezone as a Magnet Pull user. While Alolan Golem itself isn't worse, the restructuring makes it drop a rank.
A->B - Similarly to Alolan Golem, Magnezone drops a rank to make room for Zapdos. Its Magnet Pull set and even its Analytic set are both great for rounding out an Electric team, making it fit better below those in A. 
B->D - The help it provides to Tapu Koko is less relevant with Zeraora covering Steel very nicely. It fits more with the likes of Raikou, who's outclassed against Psychic by the variety of Knock Off users.
C->D - Xurkitree joins Mega Manectric and Raikou as the mono-Electric special attackers that have a very small niche but are usually not the right choice. Its ability to sweep specific teams based on coverage is offset by its reliance on Hypnosis, weakness to priority, and still mediocre Speed after it boosts.
C->Unranked - Electric has much better speed control options than webs.
D->Unranked - The last thing Electric needs is another fast special attacker, especially one that needs Agility to outspeed most relevant offensive Pokemon.
D->Unranked - There are much better options for handling Mega Sableye and it simply doesn't beat anything anymore.
Unranked->C - Rotom-C offers an alternative way of handling Tapu Bulu and has the benefit of being marginally better against Mold Breaker Excadrill for teams that require extra assistance, but it prevents the use of Rotom-W. It is, however, significantly better than the Pokemon in D.
S->A - Azumarill has become much easier to drop as Fairy chooses which matchups its willing to throw away. Azumarill covering Poison with Jolly Frustration and Steel with Spikes supported Aqua Jet is simply not as reliable as it once was, and it simply does not compare to the power of the other S ranks.
A->S - Tapu Bulu's incredible range of utility options and SD sets makes it capable of beating almost anything with the right tech. At any point in the metagame, the threat of running into the very different Disable, Leech Seed, double Grass STAB, Rockium Z, and Fightinium Z sets and how it pressures different types for Fairy to enable its other Pokemon makes it a solid S rank.
D->Unranked - The loss of Mega Diancie is far too much of a cost to pay to run Mega Gardevoir. It doesn't help that Mega Diancie is actually better vs Poison anyway.
D->Unranked - As a Sticky Web user, it is almost always worse than Ribombee, and it is always worse than Azumarill as a Belly Drum user.
D->Unranked - It's actually worse than Tapu Bulu at SubSeed even though it has Prankster, and utility sets with Tailwind and Memento just don't cut it anymore.
C->D - The opportunity cost of Mega Gallade is absolutely massive, and this should reflect that more accurately.
D->C - Toxicroak's ability to run certain sets that punish Fairy and Psychic teams is more useful than losing Mega Gallade to run a slow Pokemon like Mega Heracross.
S->A - While Torkoal's role as a very reliable spinner + rocker is huge for Fire, comparing it to Mega Charizard X is a bit ambitious.
A->B - Volcarona finds that Blacephalon's Ghost STAB is much better at beating Psychic than its Bug STAB, which is unfortunately not the greatest against Victini. It has the benefit of being better against certain types like Electric, but there are other options available for beating Electric.
B->A - Blacephalon serves as probably the most reliable way to handle Psychic, making it jump over Volcarona. It can even frustrate Flying, which is often dropping Mantine these days, and Dragon with its coverage and STAB attacks.
B->C - With Volcarona dropping, Victini drops a little too into Alolan Marowak's tier. Not necessarily a knock against Victini, but it fits better with the decent Stealth Rock user than the matchup moth.
B->D - Too many types are running Pokemon that naturally beat Mega Charizard Y in SM, and Mega Charizard X's power is too good to drop for this.
C->D - Volcanion's biggest problem is there are too many things it wants to be able to do, and that causes it to be lackluster in every role it can try to run. As a wallbreaker, it can't fit all the coverage it wants, as a Defogger, it doesn't beat many rockers, and as anything else, it lacks power. 
D->Unranked - While at least Entei has the "benefit" of powerful Extreme Speed, Darmanitan simply does not cut it anymore, as all it can really do is click Flare Blitz and U-turn with a Choice Scarf.
S->C - Mantine has fallen significantly in usage and viability recently. Like with Zapdos, BoltBeam has taken over the metagame, and Mantine just can't compete. Offensive Defoggers are much more popular on balance teams, the only place Mantine could fit, and Mantine's lack of presence outside of a measly Scald does not help its case. 
A->D - Like with Mantine, Skarmory is just too passive in the current metagame. Celesteela being a superior defensive Steel / Flying type that can double as an offensive threat as well makes Skarmory very difficult to fit into most Flying teams.
B->A - Dragonite's ability to have powerful Choice Band attacks while always threatening to be a Dragon Dance set makes both HO and balance teams love to use it wherever they can.
B->A - Tornadus-T is everywhere on fatter builds as an amazing utility option. It can run all sorts of different sets between just stallbreaker and Defogger roles. It fits on all sorts of teams, notably being on almost every bulky offense and even most balance teams.
C->B - HO teams are becoming more popular and Aerodactyl is HO's favorite suicide lead. As a result, it goes up.
A->C - Cradily's spot on the defensive core along with Mega Venusaur + Ferrothorn is much less useful in the world where Fire-types are less and less frequent. Instead, matching up and beating types that Grass is naturally good against is the prevailing strategy, so Cradily drops significantly.
C->B - Decidueye almost single-handedly gives Grass a chance against Psychic, what once was one of its worst matchups. While not completely reliable due to Psychic's love for anti-Mimikyu tech, Decidueye also helps in other matchups and can even run tech options to help check a variety of specific threats to Grass, including Alolan Muk.
S->A - Sand is no longer the undisputed best way to build Ground, and Hippowdon being such deadweight in most matchups means it's dropping from S.
A->C - There are much better ways of handling the Pokemon Dugtrio used to trap. The absolute need for removal just to make Dugtrio work means its viability has dropped a lot.
B->C - Gliscor drops simply because it fits in more with the C ranks than powerhouses like Mamoswine and Mega Garchomp, who have very notable niches as powerful wallbreakers.
B->C - Landorus-T drops slightly to better reflect the opportunity cost of Landorus. It should be closer to the Pokemon that it is often used with, as while it is still very powerful with double dance sets, the team needs to make up for the lack of Gravity.
C->A - Mega Steelix has taken the metagame by storm in the past year and finds itself on every variant of Ground between sand and sandless. Covering many matchups that Ground hated in Normal, Fairy, and Dragon is invaluable. 
C->D - Diggersby is simply not as good as the other C ranks, as Krookodile serves as a very solid answer to Psychic, and the core of Landorus-T and Nidoking are especially powerful on non-sand.
A->B - Offensive spinning with Alolan Sandslash is preferred these days.
D->Unranked - I don't even know why this is ranked or (even worse) has an analysis. Never seen it used even at 1000 ELO. My bad for letting this horrible thing stay on the VR for so long.
B->C - Dropping because the other B ranks are just much better.
C->Unranked - This thing is so bad.
S->A - While Mew still fits on every team archetype, it's hard to compare any Psychic-type to Victini, which is all but needed on every Psychic team.
A->B - With HO being much worse than early gen and balance dropping Alakazam more and more lately, Alakazam falls down the rankings a little bit. 
A->B - Jirachi is still very good, but the competitiveness of the A tier means it drops to B where it fits slightly better.
A->C - Slowbro remains one of the most iconic balance Psychic walls, but the emergence of other options has made it fall down the list of best Colbur users, and that is reflected by its drop here as well.
B->A - Celebi has risen as perhaps the best Colbur user and one of the best Stealth Rock users on Psychic. Its fantastic bulk and special typing makes it a fantastic check to Fairy. Unlike Jirachi, the other defensive Stealth Rock user, Celebi has recovery, which makes it extremely reliable for setting Stealth Rock throughout the match.
B->A - Nasty Plot Deoxys-S took the metagame by storm with its incredible coverage options in BoltBeam, Energy Ball, and of course its STAB Psycho Boost. Being able to check a variety of types with Expert Belt or a Z-Crystal, Deoxys-S fits on many teams as a reliable answer to Flying and Water.
B->A - Metagross has overtaken Jirachi as the superior Steel / Psychic type. Between its extremely powerful Choice Band set that can run all kinds of moves between Pursuit, elemental punches, STAB, and Hammer Arm, Metagross can cover almost any type offensively. Defensively, Metagross's bulk and powerful STAB attacks means it can pack a Kasib Berry and handily check Mimikyu and Decidueye. It can also run a Colbur Berry to handle Dark-types, and its versatility gives it this A rank.
B->C - While Mega Slowbro is arguably better now than it has been for quite some time, everything on Psychic is a lot better than it used to be. As a result, Mega Slowbro slides down a rank.
C->B - Mega Latios tends to be a great glue for teams that have no Mega Evolution. Unlike with Mega Latias, Mega Latios's superior damage output lets it break through teams with its Calm Mind sets. While most teams will still prefer Latios, Mega Latios's viability has gone up recently.
D->Unranked - While not entirely unusable, it shouldn't sit with Mega Alakazam and Mega Gardevoir, which have real niches.
S->A - Mega Scizor drops for the simple fact that S is currently held by Celesteela and Heatran. It's really hard for any Pokemon to compete with that, and Mega Scizor fits much nicer in the A tier.
D->C - Cobalion rises for being a really good option on HO for especially handling Dark using its SubSD set.
A->B - Gyarados is still great, but it's got way too much competition in the A rank to fit here.
A->B - The old mega fat semistall Suicune Water teams are less prevalent than before. While Suicune still threatens many types and is a very solid choice, it's hard to fit on the slightly more momentum-focused Water teams of today.
C->B - Rain teams that aren't full on 100% Swift Swim are more common, and Pelipper is the best rain setter for Water.
C->D - As a stallbreaker, there are just better options around.
Unranked->C - The infamous "Triple Degenerator" team is very successful but very matchup dependent. As a result, Slowking rises to the other stall-based Pokemon.
