I know it's a lot of drops but since VR might be updated soon I felt like sharing what I think about the metagame as a whole; note that some of these picks may be a bit biased or inaccurate, but I don't think it's super obvious (excluding the last nom of course but that's different + not a proper nom since I don't expect these guys to change anyways).
A1 --> A3 I think putting Extreme Killer above Marshadow, one of the tier's best revenge-killers and wallbreakers, is a bit of an overstatement to how good it is. Extreme Killer I feel isn't nearly as splashable as Marshadow is, only being common on Offense to Hyper Offense structures, where I've seen it still have to compete with Arceus-Ground and Ultra Necrozma for a physical sweeper, the latter having a much easier time setting up in my experience, especially with way less reliable checks, and fitting on different types of HO structures like PsySpam and Webs compared to Extreme Killer. Of course Extreme Killer isn't bad, and you could say this is a bit of a nitpick, but I just don't think it's better than Marshadow or Ultra Necrozma.
A-2 --> A6 Giratina-O is such a good mon right now it's insane. Although it's longevity isn't as good as Ho-Oh's, it is still such as good Defog user with great utility in Toxic, Will-O-Wisp and Thunder Wave. It has great Tera options in Poison, Steel and even Fairy to some extent, and can even afford to run Priority. Such a splashable mon too, and although it may not be fantastic at any of these traits, it's a jack-of-all-trades mon like Arceus-Ground. Compared to Arceus-Fairy, Giratina-O isn't nearly as passive against mons such as Zacian-C either due to its solid offenses or utility, and I feel like its much easier to justify using. Although, I am considering Arceus-Fairy to rise as well, but I think A-1 is good enough for it at the moment, again because Utility sets kinda struggle with 4MSS between Taunt, Toxic, Will-O-Wisp and even Earth Power, and offensive sets can feel underwhelming at times.
A-4 --> B+ I've expressed this before but I really don't like the inconsistency with this thing. Sure it's great into Bulky Offense and some Balance structures, but outside of that it gets destroyed by HO and does nothing against Stall. Having an inaccurate STAB and coverage options makes this feel like a gamble too often to justify putting this high in my opinion, especially since it faces so much competition with Eternatus, who's faster and has more reliable moves to use. It doesn't have that much defensive utility either, having barely any resistances, with the only noteworthy mons being Primal Kyogre (which can chip it significantly with Thunder) and Defensive Necrozma-DM (not sure if Substitute even lives Earthquake, and this thing is kinda mid anyways).
A-6 --> B+ This thing is so hard to build around and suffers from so much 4MSS it's insane. Sure, unscouted MMY is pretty terrifying to fight against, but it can't hit everything it wants to. Psystrike and Nasty Plot are mandatory, meaning it only has 2 moveslots for coverage. Ice Beam is necessary for Yveltal, Fire Blast is necessary for NDM and Focus Blast is necessary for Arceus-Dark, so whatever you run you're bound to get walled by something, and die/be forced out by said things. The first two options are also kinda mid into stall since Dondozo walls you then, which is pretty absurd for a special attacker, although Dondozo itself has to be careful with its HP to do so. Compared to Deoxys-A, who's seeing a massive surge in popularity because of PsySpam, notice how MMY has just... faded away. Deoxys-A is just faster and has the ability to fit all the coverage it needs into one moveslot thanks to Meteor Beam, or just nuke stuff with Life Orb + Tera Ghost Tera Blast. MMY is just underwhelming compared to Deoxys-A and it has very fringe cases where it's better such as on non-PsySpam Webs teams or Dual Screens HO.
B --> C+ There isn't really a reason to use Grimmsnarl over Deoxys-S in my eyes. Being unable to stop Yveltal from using Defog, especially since Defensive and Scarf Yveltal are so popular, is pretty bad. I suppose being able to get multiple screens up in one game is neat, but the Rocky Helment Deoxys-S spread can do so as well, especially against offensive teams, just like Grimmsnarl. I suppose being Dark-type is neat against PsySpam, Basculegion and Marshadow (especially since the Fairy-type helps against the latter), but these feel like niche cases rather than something to justify it being B tier.
B- --> C Listen, I agree that Life Orb Yveltal is annoying to handle. This is not a good reason why this thing is all the way in B- though. It doesn't do anything else outside of that, that other mons can't do. The only reason I'm not putting it in D tier is because Heal Bell is neat for Fat Balance teams, and Volt Switch is still useful. But apart from that, this mon doesn't do much and I'm shocked it even stayed here after Xerneas got banned.
B- --> C+ I'm just gonna link this:
Honestly drop Smeargle. I hate using this thing. It feels so inconsistent and easy to play around especially compared to any other HO lead. Deoxys-S has Magic Coat and Taunt which it can afford to run either. Grimmsnarl has Prankster Taunt which is even better. Glimmora has Mortal Spin and Mud Shot to discourage support Arceus formes and other Taunt users from staying in on it. Shuckle has Mental Herb and Mega Diancie has Magic Bounce so Taunt doesn't matter against those two.
But Smeargle is weird since it has the biggest 4MSS in the entire tier IMO. You want Spore or Nuzzle for status inflicting, Sticky Webs for the whole niche, Ceaseless Edge or Stone Axe for additional hazard support, and then you have the fourth moveslot, where Smeargle wants Mortal Spin, Rapid Spin, Whirlwind, Taunt, Magic Coat and even Icy Wind to an extent. Obviously it can't run all of it, making Smeargle fine in some MUs but miserable against others, like if I had Whirlwind to stop setup then I'm useless against Glimmora leads, but if I had Rapid Spin then I fold to setup sweepers like Refresh Arceus formes, Refresh Mega Salamence and the more niche Substitute Zekrom. Even then, compared to Shuckle and Grimmsnarl, Smeargle has no way to bring in teammates either, since it can't usually run Final Gambit or a pivoting move like Parting Shot. It doesn't have an offensive prescence at all unlike Mega Diancie or Psycho Boost Deoxys-S (even if the latter is a bit limited), and it doesn't have a very reliable ability since Moody can mess you over by reducing Speed, or make you unstoppable by boosting it instead.
Overall Smeargle is just really inconsistent and strange to use compared to the other HO leads in the tier, it's definitely better than other leads like Landorus-I but that doesn't really justify it being in the same tier as Glimmora or Shuckle, both of which are very trustworthy in setting up hazards. I argue Smeargle should be C+/C since a lot of teams that use it could justify using Shuckle instead.
C+ --> C- I had to ask what this thing even does that's how irrelevant it is. Apparently it checks physical attackers like Zygarde, Arceus-Ground, Extreme Killer and Marshadow, but I have never in my life seen this thing be used. I want to put it into UR but I know there must be something justifying it in C+ somehow. It also just looks... passive? I'm not sure what the standard moveset or spread is (because again I've barely seen anyone mention it let alone use it) but I have a feeling this thing just lets every special attacker in, especially if it's using Bulk Up. Also, what's wrong with using Zygarde? Is Roost THAT valuable? Is the Ground- and Fighting-resistances THAT useful to justify over Zygarde? I don't get it.
C+ --> B- If Giratina and Chansey can be B- for stall, why can't this thing? It's as much of a stall staple as Chansey is. It may be a bit passive but it's gargantuan bulk and Unaware more than makes up for it, vastly limiting the number of Physical mons that can somewhat break through it.
C+ --> B- This thing is seriously underrated and underestimated. Even in the tier with Primal Groudon, BoltBeam is still really strong coverage, and outside of Primal Groudon there aren't many mons that can reliably answer this thing, especially if its behind a Substitute. It smashes through Stall, does well into Fat Balance teams and can hold its own against Offense and HO teams since it is a decent switch-in to Primal Kyogre.
C+ --> C- Mewtwo can't catch a break can it? But seriously just use Marshadow or Mega Salamence for a physical attacker. Just like MMY, it struggles with 4MSS since it wants so much from Zen Headbutt, Ice Beam, Knock Off, Stone Edge, Taunt and even Poison Jab or Bulk Up, as well as it struggling to compete with NDM or Ultra Necrozma for a strong Psychic-type. There's just not enough good reasons to justify using this thing, to the point where using this thing JUST for STAB Focus Blast doesn't sound so far-fetched IMO. The only reason I'm not putting it in D tier is because I feel bad, or rather because it can be a little bit annoying to pivot around if it DOES get the coverage right, but this logic would apply to any sweeper (like Facade Mega Salamence vs a status-spam team).
C --> C+ Being a good switch-in to Yveltal, Mega Salamence and Rayquaza is pretty neat, as well as arguably being one of the most reliable Stealth Rock setters in the game being able to scare away both Ho-Oh and Defensive Yveltal from Defogging, leaving only Giratina-O to do it (another reason why it should be ranked higher). It's pretty strong too considering how solid Rock-type STAB is right now.
C --> C+ This guy is just annoying. Too annoying for C tier IMO so I'm ranking it higher. It is a great Arceus-Ground and Stone Edge Primal Groudon switch-in though, and it does have good utility in Toxic, Stealth Rock, Defog and U-Turn. Also it's probably the more consistent mon out of the all the C tiers, still finding a use in pratically any game it's in, so putting it higher does the OU legend a bit more justice I suppose.
C --> UR My opinion has not changed about this guy. Stall already runs Refresh Arceus-Dark so is there really any purpose of running Mega Sableye? Even the stall teams that run Mega Tyranitar wouldn't want this thing. A Dark-type that loses to Yveltal is also pretty pathetic, and it barely beats Marshadow too, since Choice Band Poltergeist 2HKOs, and Giratina-O can just drop a Draco Meteor on it. Just like Buzzwole, I've barely seen anyone use or discuss this thing in enough detail so I can't justify it for even C- to be honest. Well, at least it beats MMX (no it does not beat MMY it gets destroyed by +2 Ice Beam or Fire Blast).
C- --> D/UR These guys actually suck. Landorus is fast but not fast enough to truly be a threat to most teams. It struggles with 4MSS since it wants Rock Slide, Focus Blast, HP Ice, Stealth Rock, Taunt and Explosion. As an offensive hazard lead, I would just use Deoxys-A who is much more threatening in my eyes. As for Skarmory, there's not much reason to justify using over Giratina for stall teams, or even Dondozo. IronPress is not that useful or powerful in that tier and Garganacl does that better (even though its C- too), and Spikes spam isn't super important for Stall either.
C- --> C+ PsySpam is a THREAT and all the tour games should justify so. Deoxys-A, Ultra Necrozma, Basculegion, Lunala and even Zacian-C become super terrifying under Psychic Terrain which is provided by Tapu Lele, as well as it being a nuisance with Taunt and Nature's Madness, ensuring it isn't setup fodder against Zacian-C or opposing NDM. It's really popular on ladder, in tours and is generally easy to pilot, with some people even calling it braindead (which is a giga stretch but I can see why they'd say that).
D/UR --> C-? I'm not going to get into detail but all of these guys (especially the first one!!!) have unique traits that make them somewhat viable in my eyes. I don't have the time to explain every single one in detail, and I certainly don't have the time to share replays, but this is more of a theory or idea if anything else. I'll probably make separate nominations for these guys in the future, but that depends on how the metagame shifts overall. Most of these guys come from other players using them however, such as Naganadel being used in NDUbers SSNL by 09-10a, or Ursaluna-BM being mentioned throughout the NDUbers Discord. But again, I feel like these guys are either underexplored, or I'm just being silly and ranking bad mons (probably the latter, apart from the first mon of course, gdarm is goated, gdarm my beloved).
update:
252 Atk Parental Bond Kangaskhan-Mega Sucker Punch vs. 0 HP / 0 Def Necrozma-Ultra: 192-228 (57.3 - 68%) -- approx. 2HKO
is washed i take it back go back to D tier