Just did a run with these
(non trade from FR) ----->
S rank
Incredible mon. Outside of no longer hitting Gengar and Weezing with Earthquake/Dig his dominance from rby is mostly untouched, actually gets some nice perks to make up for it such as megahorn and poison point finishing off kills for you sometimes, and is basically set until the absolute endgame as soon it gets the Rock Slide tutor from Rock Tunnel. Only really bad match ups are Misty (not strong enough yet) and Misty v2.0 (without game corner tms), anything else is handled with no problems considering the options it has available. Game corner tms (mostly ice beam and thunderbolt) and some x items just to give him the boost it needs to solo all the non Lance elite 4 members + Champion might be needed to use it to its full potential against the elite 4, but by absolute late game dumping cash on them is a non issue and when Nidoking is by far the best possible user of the game corner tms anyway.
The slow start as Nidoran is not even that bad when MT. Moon can easily level him up between a bit of sharing exp with something else, bug catchers, and wild paras to make it incredibly easy to come out of there with a Nidoking. And once it gets dig (and use of the rock tomb tm) it should have no issues reaching lv 22 before SS Anne rival where it gets a pretty strong and consistent move in Thrash to complement his rock/ground coverage and carry it for a long time.
----->
S rank
This guy being anything below S would be an insult to it. This is the list of relevant matchups where Squirtle/Wartortle/Blastoise is not optimal: Surge and Erika that can be done later if you really want to anyway. Its level up movepool is mostly irrelevant outside of Bite after beating Brock, but that is all it really needs when it and Water pulse are strong enough to carry it until it can get Surf as Blastoise. Also has an incredibly easy time being higher level than the rest (at least until you get to Koga/Sylph rival/Sabrina) when Rock Tunnel, Lavender tower, part of Sylph, and Rocket Hideout are seemingly made to cater to its most natural moveset possible. Elite 4 + Champion need some x item and guard spec set up (because Blastoise gets no boosting moves) to properly sweep them, but that is hardly an issue when all 5 of their starting mons don't really do much to Blastoise anyway. Even with him soloing the elite 4 because it ended up being faster than anyone else doing it and some rare candies, this only ended at lv 58.
Used Ice beam (obtained late game when dumping cash was not a problem) on this one, but Blizzard from Cinnabar mansion with x-accuracy for Lance's dragons and Venusaur works just as well.
----->
C rank
Only bothered with this because it was the first mon that appeared in Viridian Forest lol. Anyway, this thing is not that impressive outside of natural Thunderbolt that at least make it easy to lv up, but getting there is the problem because the Pikachu stage is awful. If the Pikachu stage is skipped by getting it at the power plant, it doesn't really do much outside of bullying the swimmers, anglers, and bird keepers because thunderbolt is all it really does thanks to all its other moves sucking for it. Lorelei and Lance (this will need seismic toss or a strong normal move for the Dragonairs though) are the only relevant matchups where Raichu does anything significant.
----->
Can stay in A just fine
Being slow as fuck to guarantee it almost always will take a hit first sucks, but its bulk, power and mostly good relevant matchups make up for it. Also has utility as a mini hm slave when it only really needs normal stab and brick break to do his thing for most of the game, with late game shadow ball being an option to deal with those dumb Gengar if someone wants to do so. Being able to brutalize Sabrina and Blaine (needs thick fat for this though) while being pretty underleveled is also a really nice thing to make his horrible experience group not as bad in practice, or at least not for a bit.
:rs/Farfetch ----->
D rank
Perfectly fine where it is because it really falls off shortly after it is obtained, but being able to make up for its awful offenses with guaranteed Adamant, Stick for a good crit chance, and natural swords Dance at least saves it from being truly useless outside of fly slave. Could even got C considering that rank is filled with some really questionable things right now that and when its good traits kind of make up for how awful it is stat wise while not being that slow.
----->
D rank and could even be E because it is that bad
Now this is a real piece of shit that is way too reliant on early game corner tms to at least be remotely useful outside of killing route trainers that die to almost anything anyway. If it doesn't, it is stuck with really bad moves for a long time that make it so bad to the point Farfetch'd of all things outperforms it in the one gym where it should have an advantage. Even with game corner tms as an option it is really low on the list of thing that deserve those when there are at least 10 different things that are way better uses of such a luxury, while late game it is incredibly subpar regardless.