As promised, here's some reasoning for the recent VR changes. My bad for the delay!
Ho-Oh is a phenomenal balance staple that checks pretty much the entire metagame in some way between Sacred Fire burns, other status effects, and phazing combined with Tera Grass or Fairy. This includes the entirety of S and A bar Facade Mega Salamence. Such role compression is nothing short of jaw-dropping and turns Ho-Oh into a nigh undroppable and highly customizable glue. Ho-Oh is also the premier Defog user in the metagame, and while it may not necessarily be the best one [loses to Stone Edge Primal Groudon], freeing up the Defog slot from your Arceus form can amp up your team's threat level significantly. It deservedly rises to S-.
Xerneas and Zygarde-C are the scariest win conditions in the metagame; their threat level is sky-high when harnessing Terastal. Xerneas obviously needs no introduction at all, having taken the metagame by storm since day 1 with its Geomancy Tera Electric set. With this, Xerneas can flip the matchup against its two best checks in Necrozma-DM and Ho-Oh, none of which can stand up to +2 STAB Thunder. This is reflected in their favoured Terastal typing, with both favouring Tera Grass at the moment. Zygarde-C rises due to the emergence of the DD + Thousand Waves set. With this, Zygarde-C is able to trap traditional checks in support Arceus formes, Terastalize into a neutral / favourable typing, boost up to +6 and sweep. It does all of this while also hard-checking the best Pokemon in the game in Primal Groudon, which lets it fit into your typical balance teams nicely. Both Xerneas & Zygarde-C have proven to be a cut above your average setup sweeper. To reflect this distinguishment, we are raising them both to S-.
Arceus is a staple on all sorts of hyper offense teams as a dangerous setup sweeper that takes advantage of Terastal beautifully, with SD Tera Normal and DD + Taunt Tera Ghost sets being some of the best late-game cleaners in the metagame. It takes full advantage of Marshadow and Zacian-C's relative fall from grace compared to prior generations. Arceus' threat level is more befitting of being ranked next to fellow top-tier sweepers in Mega Salamence and Zacian-C.
Arceus-Dark counters DD Ultra Necrozma, while also being a fine check to Primal Groudon, Mega Salamence, and Necrozma-DM. What truly separates it from other support Arceus formes though, is access to STAB Foul Play. This move is a literal godsend for it, letting it punish Koraidon and Zacian-C, and allowing it to blanket check the myriad of setup sweepers in this tier. Arceus-Dark benefits hugely from Terastal as well, with Tera Poison letting it flip the tables entirely on its nemeses in Koraidon, Xerneas, and Zacian-C. A simple Foul Play / Taunt / Toxic set does all of the aforementioned, while turning it into a great win condition. It ranks as the best support Arceus form in the current metagame.
Marshadow and Deoxys-A are very respectable wallbreakers with Tera Ghost. Marshadow has a much easier time breaking through its traditional checks, with Choice Band Tera Ghost Poltergeist 2HKO'ing the bulkiest of Arceus-Fairy without any prior chip damage required. Due to the widespread coverage of Ghost as an offensive type, a strong Ghost move is hard to respond to properly, even with defensive use of Terastal. Obviously, no Ghost resists want anything to do with Close Combat either. Additional utility in Shadow Sneak and Pursuit allows Marshadow to revenge kill weakened boosted threats, distinguishing itself from wallbreaker Koraidon. Marshadow's days as universal speed control are over due to the redefinition in speed tiers at the hands of Koraidon, Zacian-C, and Dragon Dance Arceus formes, so it is unlikely Marshadow will be able to reclaim its former glory, but we believe a rise to A- is fair to reflect its capabilities as a very self-sufficient wallbreaker.
Deoxys-A finally got the tools to overcome some of its biggest roadblocks in Necrozma-DM and Extreme Speed; Tera Ghost Shadow Ball. Combined with its clutch speed tier outpacing both Koraidon and Zacian-C, this turns Deoxys-A into an unstoppable breaker in the right hands. Life Orb Psycho Boost hits like a truck, and when combined with nigh unresisted coverage courtesy of Shadow Ball and Low Kick, any traditional switch-in to Psychic-types has to think twice. The fourth slot can be chosen based on team-specific demands, with Ice Beam denting Yveltal and Zygarde-C, Rock Slide luring both Yveltal and Ho-Oh, and Extreme Speed adding handy utility against setup sweepers. All in all a very scary threat that is only held back due to high skill requirements to make the most out of it.
Flutter Mane's broken type combination and great special bulk lets it come a long way in a metagame filled with bulky Dark types and special attacking support Arceus formes. This gives it plenty of opportunities to set up Calm Mind and clean up weakened teams between Shadow Ball and Moonblast combined with just the right coverage move in Power Gem to smack Ho-Oh. Flutter Mane is also the only offensive threat with a positive matchup against all Koraidon sets, immediately giving it a boost in a metagame that is dominated with a capital D by Koraidon. It's a nifty anti-meta threat that scores itself a raise due to all of these reasons.
Palkia's Origin form is a vicious anti-meta wallbreaker capable of taking advantage of tier superglues in Necrozma-DM and Ho-Oh with Substitute. From there, it can freely fire off Lustrous Globe- and potentially Tera Water-boosted Hydro Pump to drown a large swath of the defensive metagame, with Spacial Rend in the back to 2HKO the bulkiest of Primal Groudon after minimal chip and OHKO most other Water-resists. Not even Ferrothorn is safe, as it is taken down by Fire Blast. Such wallbreaking prowess on a Pokemon with a Speed rivaling that of Arceus and Mega Salamence ought to be respected, hence the rise to B+.
Dual Screens has been taken to a new level entirely this generation, courtesy of Grimmsnarl's newfound access to Parting Shot. With this move, Grimmsnarl is able to create lose / lose situations for the opponent, as even if you manage to remove Screens with Defog, Grimmsnarl remains capable of weakening you with Prankster Parting Shot and enabling a setup sweeper in the process. Defog Arceus-Dark and Yveltal are the only Pokemon capable of avoiding this interaction altogether, but neither appreciate Play Rough draining their Recover / Roost PP. Combine this with the fact that Screens Hyper Offense is the most resistant to Choice Scarf Koraidon / Ditto as a hyper offensive playstyle, and it shouldn't come as a surprise why we're raising Grimmsnarl.
Arceus-Poison compresses a check to top tier staples in Koraidon, Xerneas, Zacian-C and Arceus, while also giving you a Toxic immunity and Toxic Spikes absorber. You do have to make sure you couple it up with checks to Primal Groudon and Zygarde-C, since usually you would use your Arceus slot to check those. This isn't much of an issue however, since you can just pair it with the likes of Coil Zygarde-C, bulky Primal Kyogre, Giratina-O etc. and you'll be fine. A severely underrated Pokemon that rises deservedly.
On the other hand, Arceus-Dragon compresses a check to a unique group of Pokemon in Primal Groudon, Zygarde-C, Mega Salamence, and Primal Kyogre, while boasting a favourable Judgment type into Koraidon, letting it serve as a soft-check to it. Its ability to annoy Primal Kyogre is of particular importance, letting it distinguish itself from the likes of Arceus-Fairy and Arceus-Dark. It rises a subrank to better reflect its overall utility.
Mega Mewtwo X thrives in a metagame where the primary Fighting-type resist is Ho-Oh, which can't switch in to Stone Edge obviously. The popularity of Arceus-Dark and Ho-Oh gives Mega Mewtwo X plenty of opportunities to break walls, making it a fairly effective anti-meta option. It can't break Arceus-Fairy as reliably of course, but can still pressure it well enough with Poison Jab depleting Recover over the course of a game. It will be placed in B- for now as a decent option to overwhelm the defensive integrity of a team when used alongside fellow Fighting-types in Koraidon and Marshadow.
Mega Blastoise carves out a niche for itself on hyper offensive teams as a setup sweeper capable of threatening the likes of Ho-Oh and Necrozma-DM. Even Primal Groudon drops to +2 Dragon Pulse after some chip, which makes Mega Blastoise a fine cleaner situationally. However, its very weak to priority and Scarfers, so a modest raise to reflect that it has a niche ought to suffice.
to C+, C, and C-, respectively
Buzzwole, Heatran, and Garganacl all manage to secure themselves a spot on the VR. Buzzwole does so by being a fine check to the likes of Zygarde-C, Arceus, and other physical Arceus formes lacking Taunt. Heatran dominates Ho-Oh, Eternatus, Arceus-Fairy, and Toxic Necrozma-DM, making it a deceptively good balance breaker. Similarly, Garganacl dominates Ho-Oh while serving as a status absorber and win condition between Salt Cure, Iron Defense, and Body Press.
Arceus-Steel is very hard to fit as an offensive Calm Mind user due to the prevalence of both Ho-Oh and bulky Primal Groudon. While it can run coverage for them in the form of Power Gem and Earth Power, it only has place for one of these coverage moves. You can drop Recover on hyper offensive teams, but at that point you are probably much better off running Calm Mind Arceus-Ground. Its weakness to common revenge killers doesn't help it out much either.
Gothitelle is a very matchup-dependant Pokemon, and it just so happens to not like current matchups at all. The most common Arceus form is Arceus-Dark, many support Arceus forms run Taunt, and hyper offense is very prevalent. It can still trap the likes of Necrozma-DM and a Choice-locked Koraidon, but tends to be deadweight in way too many matchups for this to be worth much.
The Landorus formes are mostly outclassed in this metagame. Landorus-T is a niche check to the likes of Primal Groudon and SD / DD Arceus formes, but it tends to get worn down / overwhelmed quite quickly. Landorus is a niche suicide lead on Sticky Web teams that can hit quite hard, but doesn't have a lot going for it over Stealth Rock + Eruption Primal Groudon other than Explosion.
Bulkier teams continue to suffer under a constricted metagame dominated by the nigh unwallable Koraidon and the myriad of Terastal sweepers / general setup sweepers. The recovery nerf was a huge hit as well, further diminishing these teams in longer games. Chansey, Blissey, Mega Sableye, Mega Slowbro, Clodsire, Toxapex, and Skarmory are some of the poster children of these bulkier teams, and drop as a result.
Celesteela drops as a result of Ho-Oh's dominance. It can barely make any meaningful progress in this metagame.
A host of niche Pokemon drop even further due to the centralized nature of the metagame. Excadrill and Cloyster are rarely worth it over Deoxys-S. Access to Rapid Spin doesn't mean a lot when the metagame's most common entry hazard setters are shut down by Taunt already. Mega Blaziken is almost never worth it over SD Koraidon. Regieleki struggles with Primal Groudon's dominance over the metagame, with Tera Blast Ice not doing a whole lot to bulkier variants. This generally makes it inferior to the countless other wallbreakers in the metagame.
None of these Pokemon hold a tangible niche anymore. Chien-Pao and Urshifu just don't hit hard enough to warrant usage over other breakers. Choice Scarf Koraidon and Ditto vastly outclass Galarian Darmanitan. Roaring Moon isn't justifiable over other Dragon Dancers. Mewtwo doesn't really have anything going for it anymore; Taunt sets are useless with the recovery nerf, and offensive sets are outclassed by Mega Mewtwo Y. Arceus-Electric's niche as a Calm Mind sweeper that keeps Ho-Oh from using Defog is worthless now, with better Calm Mind Arceus formes now having access to Power Gem.
Dugtrio and Wobbuffet aren't worth using; the things they trap can also be overwhelmed by other Pokemon that aren't deadweight into everything else. Tyranitar is wholly outclassed by Arceus-Dark and Yveltal. Corviknight is too passive to warrant a slot over Ho-Oh or even Skarmory.