I'll talk about most of the Pokemon in each tier, but won't go into as much depth for the lower tiers.
S Rank
Dragapult
Mew
Iron Bundle
Pult I still think is the best mon in the game.Good on offense obviously for its solid damage output and speed tier, but I think its at its best on balance teams where it can u-turn and spam wisp with hazards up, and serve as a great revenge option if needed.
Mew will always be good simply because it learns nearly every move in the game. Strong bulk on balance teams as well. My favorite set is knock, wisp, spikes, and softboiled. There are not that many good spikers so that + knock and wisp makes it so good.
Iron Bundle is one of the most threatening Pokemon from a purely offensive standpoint. Has a similarly great speed tier where it really just has to worry about Pult (it eats pult draco but can't OHKO back either). Very few Pokemon in the tier can wall it, and even less after 1-2 flip turns forcing them out.
A
Melmetal
Blacephalon
Blissey
Chi Yu
Heatran
Latias
Terrakion
Melm is great and can demolish unprepared teams. Great physical wall with Balloon and eats basically every unboosted CC and can usually farm back with twave+dib. Best set is still sleep talk + rest + twave + bash with balloon.
Clown op, great on offense, stabs are almost impossible to switch into. Steels and ghosts like to switch into booms but get hit super effective by the clown. That being said, less dominant with garg which walls it and eats boom. Chi Yu similarly tough to switch into but without the threat of boom. Things like bliss and fini wall indefinitely. Plot flame charge I think is its only really solid set. Probably around the A- to B+ range though.
Bliss is so good and on basically every stall team and a good number of balance teams. Obviously soft boiled is a given, but gen 9 mg2 has given a strong rise to aromatherapy blissey usage. I'll admit I don't know the best attacking move for it. Walled by ghosts in mg2 is rough but I still think toss is best. Ibeam is really nice vs some things but losing a 1v1 to taunt bundle basically means your team gets farmed.
Heatran is AWESOME. Use tran. Teams basically need a fire type to break through all the balloon steel types that can also eat (or be immune to) fighting type attacks. Rocks usage is great, wisp usage is great, access to boom is great, taunt isn't bad, roar isn't bad. Can choose from fire blast, lava plume, and magma storm (with either taunt or boom for bliss and stuff).
Latias is solid. Not too much to say on this mon. CM usage has dropped across the board in gen 9 so its tough to place this mon right now. It's not bad though.
Terrakion is always good. Can only be checked by a few things and they don't like coming in multiple times. CC + Sedge are given, SD basically mandatory. Last can flex into air slash (fighting type terrak checks), earthquake (poisons, knocked balloon, steels you dont want -1 for), megahorn (ass but for psychics like lati which aren't used as much) and rock polish (good mu vs offensive stuff that can't switch in as well but tries to revenge).
B
Explanations will be shorter outside of the ape
Ape is really good, certainly will be moving to A if not S rank after the tour. Can run three sets but only one is really good. BU CC Shadow ball rest is certainly the best set, very little switches in at +1 and nothing can OHKO it with its great bulk. Solid enough speed tier to outrun the fat stuff that can eat, and bulky enough to eat all the stuff faster than it. Can also run a less threatening sleep talk over bulk up set which isn't great but can still apply pressure. Last set is utility with rocks + uturn which is a nice combo to scare them but theres some better mons for it. Probably only usable on bulky offense.
Annihilape
Ferrothorn
Garchomp
Garganacl
Gastrodon
Gholdengo
Latios
Metagross (Mega)
Skarmory
Slowbro Galar (Mega)
Sneasler
Starmie
Toxapex
Iron Treads
Great Tusk
Walking Wake
Ferro + Chomp + gastro + Skarm
all solid spikers, basically choosing between these + mew if you want spikes. All solidly bulky and good on all sorts of teams.
Sneasler good pivot mon and not great stabs to switch into late with hazards but alot of teams have answers.
Toxapex + Gholdengo + Garganacl
just really solid walls. Gho is why you need a fire type. Also psychic for ape and sneasler is cute. Pex and gho are better walls but both are momentum sinks while garg's salt cure sucks to switch into.
Great Tusk, Iron Treads, Starmie
the three good spinners. Treads is probably the best and best against fat teams, starmie finen against both, and tusk solid vs more offensive while being on more offensive teams.
Slowbro-Galar + Metagross
two mons that mega and setup with CM and Cosmic Power respectively. Needs to be considered though mgross usage is basically 0 right now.
Latios just a worse latias basically but finds a niche in doing alot more to ape while still eating +1 sball.
Walky Wake is really good vs offensive teams but can just get sat on by blissey + it has limited pp. chooses from stabs + rest + stalk/flamethrower
C
Even shorter, just a brief bit about what they do
taunt + cm + recover with nice speed tier
heal bell that checks bundle without bliss momentum sink with uturn
plot rocks fblast boom
decent semi check vs fat stuff. only 3 good moves
recovery + tspikes
skarm without spikes but with pivot
shed tail (bad)
spinner that ghosts dont switch in on + spinblocker
mega or not mega, one of the few viable faries, spikes + rocks, can even hbell
basically bad treads now, no knock and worse bulk but gets SD
can go shiftegear with flamethrower and other coverage + boom
mega with plot (no spa stabs) lets you do a nice 80 chip to every mon in the tier at +2 and get ohkod back
rocks, uturn, knock, roost, SD, underrated but still probably C or C+
spikes taunt and not great stabs to switch into. good fast lead.
rocks ww recovery. too many balloons and no knock though + no sand
basically no CM users and even less psychic types + chi yu sends doom to the bin. not a bad mon but meta no longer needs it
really scary mon, might go up to B although very little usage. recovery and surprisingly bulky but also access to scary plot sets that nothing really wants to switch into.
checks a good number of physical threats in the tier but no great recovery and no physical electric
underrated, I spam this mon alot. picks up tspikes with boots on it, uturn, recovery, scary powerful fire blast, psychic as well is good.
recovery, wisp, spinblock basically
not great not bad, feels like a less threatening terrak despite a better speed tier.
gets up rocks and good mu vs some teams
bundle check, not as resilient as cuno but dtail and sub can have good mu's as well
knock + rocks +turn
less psychics means less of a niche, power creep also doesn't help. fplay + knock + turn
SD + phys stabs is nice, probs needs sub. not fast enough in a pult meta. good breaker but cant switch out cause mega mechanics
scald ibeam hypnosis recover. sits on some stuff but basically NU right now. probably just better options.
i think its trash and would put it UR but SOME council members like it cause it has suit (unviable move) and knock
recovery and rocks + good vs the good spinners but just better options. probably gonna be UR. not enough bulk.
good electric, can use CM sets but i think best is just roar with stalk.
mediocre. grounds not as mandatory so it can rarely be threatening. basically clicks electric move or boom. can spin but not good.
gets up spikes and honestly not many common leads want to deal with it.
recovery and chilly reception for pivot
decent spa check for some mons but curse sets arent the best (especially with ape running around) but has boom
less strong walky wake and not bulky enough as something like the next mon on the list
probably a C+ B- mon, fini has great typing and basically just runs stalk with scald moonblast. Checks a REALLY good number of scary threats in the tier.
spinner that also has knock and tspikes
no ability and no recovery. not great but good stats.
less need for stab fplay and suit. rocks though...
least useful of the 3 main hbell users, least recovery pp, uturn weakness, stab fplay less relevant etc
sounds great on paper with great stabs and uturn but then they just outspeed or go bliss :skull:
good stabs + boom
electrics aren't THAT good imo + no static but uturn is nice. doesn't like needing to choose between tbolt and thunder though...
bu and stabs is nice
same as zap but no recovery but has good speed tier.