S
Arceus-Normal - This pokemon is so good at what it does. It is arguably the best end-game sweeper in Ubers. It basically has only minor variations (Lum Berry, Recover, EV spreads) on a single set, but that set is very difficult to stop effectively due to its sheer bulk and power, and those minor variations change the way checks must interact with it. It is also an excellent revenge-killer, allowing it to have some defensive utility even though it isn't a true wall. Arceus-Normal doesn't particularly require team support. All it needs is that perfect opportunity where the opponent has been locked into a coverage move or has pulled the trigger on Draco Meteor, and then it sets up for free, and perhaps some slight weakening due to entry hazards and the like. Once set up it will go untouched by the majority of the tier due to the power and speed of Extremespeed and will probably tank an attack even if something does manage to touch it. The opportunity cost of not being able to use other Arceus forms and is about as negligible as the opportunity cost of not being able to use a different pokemon in the slot.
Xerneas - Essentially same reasoning as Arceus-Normal: easy to set up and dominant once the free turn has been given. Xerneas requires very little support to function, and forces switches like none other to help create offensive momentum. It has a fairly shallow offensive movepool, but Moonblast is a powerful STAB and the coverage options are good enough to make it somewhat customizable. Xern also has great defenses that allow it to tank hits from and kill the majority of would-be checks and allow it to check some pokemon (especially Mega-Mewtwo X, Blaziken, Yveltal) itself if necessary. Unfortunately, Aegislash is essentially a hard counter to the Geomancy set and Xerneas only has one opportunity to sweep the opposing team. However, Xerneas still more or less forces opponents to have Aegislash or run the risk of having more shaky checks outmaneuvered, and although Aegislash has excellent survivability, it has no reliable recovery and invites in (and gives free turns to) a whole host of pokemon that can take advantage of it, especially Ho-oh. Although Power Herb's single usage can be annoying at times, unboosted Xerneas still hits like a truck andsets other than Geomancy (especially those including Aromatherapy) are somewhat viable and potentially very threatening, providing some extra diversity that can make things even more difficult.
Kyogre - Permanent rain or not, there are very few pokemon that can tank hits and dish them out as well as Kyogre. Not only does Kyogre need no setup or support, the rain it creates passively provides support to others. Although it doesn't provide the team support as well as it used to, it is still a very powerful threat with few viable checks regardless of its ability to also provide team support. Its bulk and resistances allow it to switch in easily, and its great power immediately puts a great deal of pressure on the opponent. Kyogre is best known for its Choice sets. Although Kyogre has a fairly shallow offensive movepool, the threat of a powerful boosted water attack and coverage moves that can punish the few checks that exist are enough to create huge offensive momentum, and although these sets have neither recovery nor set-up moves they are more than capable of checking threats or sweeping. The Scarf set can also adequately revenge kill, while the Specs set is a ridiculous wall breaker. Palkia and Giratina are the only viable pokes that can switch in on both of these sets adequately, but neither of them can weather the storm for long with their shaky recovery. Beyond the versatile choice sets, defensive sets featuring Rest, Scald and/or Calm Mind can also pose as late-game sweepers or check various threats. Scald and Rest can allow Ogre to stall out some potential checks such as Ferrothorn or some Palkia variants with ease.
A+
Ho-oh - I think that few can deny that Ho-oh has the raw bulk and power to be S-rank, with few viable checks and the ability to easily burn the few that exist. It is pretty obvious that access to high-powered stab moves with excellent coverage that can also burn to play into Ho-oh's great bulk is insane, and great passive and active recovery allow Ho-oh to be played aggressively or defensively without loss of longevity. Unfortunately, the need for Defog support prevents Ho-oh from attaining the S-rank it likely deserves. It would seem that Defog support is fairly minimal help to provide, but ironically the need to run a solid Ho-oh answer like Arceus-Rock also diminishes the power of your own Ho-oh, as Arceus-Rock is often not the most reliable of Defoggers. sure, if you want to use a different defensive Arceus form you can and it will keep hazards clear pretty reliably, but it will likely invite in the opponent's Ho-oh in the majority of cases, which is a dangerous prospect. The need for a good Defogger is a very real opportunity cost that prevents an S-rank, but I think it is appropriate to give Ho-oh the next best thing.
More to come