(posted because Mark K said he'd post his changes later)
devised a preliminary list, feel free to advise my changes as they are very early in
Zygarde: S (Incredible diversity and ease of sweep with the Altarianite sets)
Groudon-Primal: S (Alike to Zygarde, but is more defensive and supportive in its potential roles) (find A+ mons when?)
Zapdos: A (Altarianite hit this mon quite hard, but it still checks and supports the team with momentum and anti-hazard support)
Kartana: A (Not as metabreaking as it used to be, Aggronite makes almost anything wall Kartana, still one of the best physical attackers in the meta though. Whenever Chloe decides to free blazikenite, it'll probably rise up to A+, or maybe even S)
Magearna: A+ (Incredible diversity in its sets and the ability to blanket check the majority of setup in the tier, as well as the majority of special attackers make this mon a fantastic glue and the removal of all Xerneas problems you ever have. Moved to A+, CM Split Cameruptite sets are extremely effective at stalling out opposing defensive cores and acting as a wincon, making Magearna an incredible pick for any team.)
Arceus-Fairy: A (See below, these two are as good as supportceus get)
Arceus-Ground: A (Standard Supportceus, it's good and reliable, fits everywhere, great mon)
Deoxys-Speed: A (Sets up Stealth Rock every damn game if a Gyaradosite variant is present, and will often set up Spikes if the opponent doesn’t have a Magearna on their side)
Gengar: A (after 22nd of September, however it is as it was on the old VR for now, as it'll get access to Zap Cannon to compliment Hex.)
Keldeo: A (An incredibly powerful breaker that defeats most Supportceus with just its STAB moves, and another claimer of the title of 2hkoing PDon. Taunt sets can also shut down traditional counters such as Toxapex.)
Blissey: A+ (As good as it gets sp.def wise, this thing tanks hits from anything, can cripple several mons, set up rocks, heal allies and status or just straight out stall them)
Tapu Lele: A (The Diancite stone has made having a Steel-type on the team fairly mandatory. However, piss poor defences and the limited life of terrain make this Pokemon a one-shot chance. Changed rank to A...)
Cresselia: B (Very limited recovery and a not so great typing make Cress weaker than other more reliable and useful walls like Zapdos, Supportceus or Blissey, still a good setup sweeper and can still take hits)
Xerneas: A- (It’s fucking Xerneas.)
Manaphy: A- (Unfortunately, the release of Diancite and Altarianite have made this pixie’s life very difficult, as it finds itself commonly revenge killed by the majority of teams. It also finds fewer setup opportunities, due to the more offensively inclined metagame.)
Raikou: A- (A- fits it perfectly, it destroys anything that's slower than it with the right set, or it can be pivoted out of a bad matchup, speed sucks though)
Toxapex: A- (Latiasite gave it life, and now it acts as an incredibly good check to the Fighting-types in the tier. Oh, and it counters the two Fire/Fairies as well!)
Entei: A- (Zygarde can kinda screw this mon over, but Sacred Fire and a very strong 155 Attack Pixilate Extremespeed can be very hard to deal with even at neutral.)
Arceus-Ghost: B+ (I've rarely seen it get any use but seems easily walled by Blissey or Aggronite Buzzwole or any WoW user not named Giratina)
Weavile: A- (I don't think this is as overrated as people make it seem, Weavile is still a very powerful late game sweeper, it can setup, pursuit trap, or just use faster priority to take down opposing wincons like nothing else does, it's a must on HO teams and also has some versatility because aside from glalitite, it can also utilize stones such as Pinsirite or Altarianite)
Arceus-Water: B- (Great defensive typing, but easily overshadowed by the other Supportceus, Will-O-Wisp variant might have some use though)
Buzzwole: A- (With Aggronite this thing becomes able to wall anything that tries to use physical damage, like really, aside from Entei, no physical attacker is breaking through this thing (Unless you use Coil Zygarde, but you shouldn't use Coil Zygarde so whatever))
Mimikyu: B- (Meh, not as good as other pokemon in B+, very rare sight in respectable player's teams)
Skarmory: B+ (Entei isn’t seen much anymore, and most variants are quite easily defeated due to the rise of Electric types and more Red Orb users popping up. However, the mix of hazard and defog support cannot be denied.)
Tapu Koko: B+ (A bit stuck on this one. Open to rising it to A-.)
Togekiss: B+ (See: Mark K)
Terrakion: A- (While teams are more ready to take on this thing, when used carefully it can still break teams apart with the right set, though it can no longer just threaten a whole team when it's on the field)
Landorus-Therian: B+ (I’ll need more experience with this mon to say more about it, but I do think that this is B+ material.)
Golisopod: B (See: Ruins of Alpha/Highlord)
(not sure where Zygarde-Complete lands)
Genesect: B (opportunity cost sucks but +1 Techno Blast hurts like anything, and Explosion is basically a 1-1 trade. Open to moving this Pokemon to B+.)
Yveltal: B (Altarianite fucked this mon big time.) (not sure where Arcanine lands either. Haven’t seen it on ladder at all, but I’m sure it has a lot of usability.)
Excadrill: B (Ability to defeat common HO leads and also set up Stealth Rocks in the face of Magic Bounce makes this mon a very good anti-meta Pokemon. However, I am yet to experiment with –atespin sets.)
Ho-oh: B (Unsure, but I’ve seen Banded Ho-oh put in a lot of work vs other teams.)
Lunala: B (Whilst the Scarf variant admittedly sucks, the Specs variant is incredibly tough for offense to wall, 2HKOing the majority of Pokemon other than specially defensive Zapdos, Magearna and Blissey. However, the common usage of these Pokemon and Lunala’s low speed tier doesn’t really put it that high.)
Volcarona: B (The fall of Pinsirite has made this mon inadvertedly better and a lot tougher to take down after a +1 boost. Bulkarona sets (Cameruptite) have also made a splash, and can hit much harder with their coverage. Open to moving this within the whole of B.)
Xurkitree: B (Won me the only game of OMPL, so sure, I’ll take this)
Arceus: B- (EKiller is dead. However, I am sure that it still has a niche in MnM. Open to moving it to C+.)
Rayquaza: B-? (Really unsure about this one. Altarianite nerfs it… maybe???)
Mantine: B- (Also unsure.)
Metagross: B- (Fairly good offensive SR lead on HO teams. Doesn’t really do much else other than take lives 1-1 however. Open to moving this mon anywhere B and lower.)
Mew: B+ (This is a blank canvas, you do whatever you want with it and it fits just about any support need your team might have, it's overshadowed by some other mons who can fulfill some stuff Mew can, but this thing can pack it all in just one set)
Shuckle: B- (Shuckle.)
Thundurus: B+ (I had the chance to play around it in the suspect ladder, and it's honestly a great stallbreaker, similar to Raikou, it destroys anything slower)
Kyogre-Primal: B (B- is too harsh for Pogre, this thing can tank a few hits and pack a very hard hitting punch, can also setup sweep with CM + Sub or CM + Rest)
Venusaur: B- (Has some use, can be a nightmare to unprepared teams, can even break through Blissey if played right. Roserade is shit though, C at max)
Garchomp: C+? (Also really unsure about this mon. Does it still have a niche?
Giratina: B+ (Great wall, great utility, walls a lot of shit now that Marshadow is gone, could even rise up if players realize just how strong this thing is)
Muk-A: Unrank? (as for now, keep it at C, I need more evidence)
Arceus-Steel: C
Arceus-Rock: Unranked (Pinsirite is way less now, there are much better Arceus forms)
Cobalion: B- (It works really well pivoting with the Diancite set, quite underwhelming when it comes to raw power though)
Nihilego: C (Latiasite gives Nihilego a much needed immunity, but a boosted Zygarde can easily take one down. Unsure about this nomination.
Solgaleo: C- (A Z-splash set on webs is incredibly lethal… if it can set up.)
Kyogre: C- (Only lethal in the right circumstances, but it can work exceptionally well vs weatherless teams.
Marshadow: C (Seems like a fairly good breaker with Marshadium Z, but both sets fall flat due to the higher speed tier of the metagame. Changed to C-. Changed again to C.)
Unranks: Vaporeon, Shedinja, Arceus-Dark (fairy-types really screwed this mon over)
Salamence: C (It's honestly as good as the other B- threats, it can be very threatening with Dragon Dance but sadly loses to anything using pixiespeed)