S Rank
The titans of the game. They bear massive offensive and defensive capabilities. Pokemon in this tier are extremely diverse, capable of fulfilling multiple roles on both defensive and offensive ends. There is virtually zero opportunity cost in using these Pokemon. They require no support from the team and have immense utility to offer.
- Ho-Oh- Incredibly strong and versatile, at the same time checking or countering half of the metagame. Pokemon gain niches simply because they can check Ho-Oh. For example, Arceus-Rock is an otherwise inferior Arceus form outside of countering Ho-Oh. (Even though it takes ~80 percent from Banded Earthquake) You could be using a threat like Arceus-Normal, a devastating Calm Mind user like Arceus-Electric, a solid Yveltal counter like Arceus-Fairy, or even Arceus-Grass, who comfortably takes on all relevant weather starters (fuck you Abomasnow). Ho-Oh can run many different effective sets- Physically Defensive, which is incredibly annoying to break through and can even live random Stone Edges from Support Arceus forms. It can also run a Life Orb set, with Sub+3 attacks or SubRoost. Another set is what I will just call Speed Booster (uses either Flame Charge or Tailwind to clean late-game). Hell, you can run Choice Banded Ho-Oh, which hits like a truck due to the high base power of it's STAB moves. Ho-Oh has influenced the metagame to the point where many Pokemon are running random Rock moves as coverage, ie. Specs Power Gem Dialga, Arceus-Grass running Stone Edge, etc. Ho-Oh also checks/counters half of the metagame provided Rocks are not up (fairly easy to accomplish, given the Defog buff). It can check most variants of Xerneas, Blaziken, Taunt+BU MMX, Yveltal, most Arceus formes, Darkrai (if sleep talk), Aegislash,
Ok so while I do agree Ho-oh is awesome and what not, I really don't know if it should get S rank. IMO its a very strong S rank mon, but the stealth rock weakness (yes it was lessened with defog but you still have to run defog whenever you run ho-oh (Defog is by no means mandatory support for a team)) is something you just have to get rid of to make it, well, viable. Without this support it simply can't offer great utility or use to a team but given this its truly a terror ;_;. It gets 2HKOed by too many things and can't abuse regenerator due to the fact it loses 50% of its HP just switching in. You can't check Xerneas or Arceus Norm or Blaziken because your OHKOed after it.
Admittedly, Defog isn't that much support to provide at all (its not huge / big and other team members do enjoy it) but it still is support crucial to running ho-oh. Ho-oh requires support to function as a tank that takes hits wonderfuly and lashes out status.
Now with this being said if we take a look at the A rank description:
Reserved for Pokemon with impressive offensive or defensive capability. These Pokemon are typically the first to be considered to fulfill certain roles and have very little opportunity cost. They require little support to function and may have significant utility.
Ho-oh seems to be a much better fit.
I'm going to go ahead and nominate Mewtwo for S/A (preferably S) rank. Since we're cramming all Mega's into one mon its extremely diverse. Mega Mewtwo X has the bulk to setup Bulk Ups infront of stuff like Groudon (it can make sure it doesn't do anything like phaze or status it with Taunt. Mega Mewtwo Y's speed is high enough to clean teams up weakened lategame, both can take STAB moves if needed. You really can't tell what set Mewtwo is running when it first switches in.
It has pretty much no opportunity cost for being a stall breaker as the only other thing you'd consider for the job is Yveltal who is way slower.
Mewtwo doesn't need much support to function either, its supposed to stall break for an enemy team, and all you need to do for it to work is to get it in on the enemy wall you want to torment.
If it gets kicked from S for subpar defensive capabilities (face it its no wall) I do believe A rank (once high / low / medium comes out High A or low S idk) is another nice fit.
Xerneas A. It is easy to hard counter and teams are prepared for its geomancy set, but is a capable late-game sweeper. Also a reasonably good cleric, probably the best in the tier. Also otherwise versatile, with various other useful offensive sets. Not the best poke in the tier but can destroy unprepared teams with fair ease.
I think Xerneas better fits S, its probably the most versatile pokemon this generation, and that's in part due to the fact its got a wonderful typing and base stats that let it take on basically whatever role it wants (no seriously it can become a wall that checks special OR offensive dragons (cleric) or a fast speedy sweeper (geomancy) or a lure / thing that spams its ludicrously powerful moonblast and runs moves to defeat counters or a Calm Mind sweeper that can setup more then once and there's even more variations from there).
Its got some dedicated hard counters in Aegislash, but most of those lack reliable recovery and in fact can be beaten by another set. There lack of recovery makes it possible for Xerneas to do stuff like hit them with a strong move and retreat.
If your talking about random mons made viable look no further then Xerneas whose helped made Aegislash / Amoonguss look legitimate since they can beat certain variations of Xerneas. ChestoRest Xerneas actually bypasses opposing Aegislash / Ho-oh / Chansey / Blissey / Bronzong / other steel types used to tank xern except metagross.
If we take a look at the what the post defines as S rank:
The titans of the game. They bear massive offensive and defensive capabilities. Pokemon in this tier are extremely diverse, capable of fulfilling multiple roles on both defensive and offensive ends. There is virtually zero opportunity cost in using these Pokemon. They require no support from the team and have immense utility to offer.
Xerneas already meets two of those.
Opportunity cost has been an issue for Xerneas sort of (I mean why use Xerneas as a cleric when stuff like Sylveon / Clefable / Chansey exists or use Geomancy Xerneas when I could use something like Extremekiller?), but its typing and bases come into play again. Xerneas has the second highest BST in terms of fairies (losing only to Arceus-Fairy), and being a fairy-type gives it the edge when considering anything else to use, due to the fact its a great defensive AND offensive typing that literally nothing but Arceus can very viably use in Ubers. This pretty much results in an extremely high usage (hype is in play sorta but its typing really heavily reduces its opportunity cost since no other fairy has such raw stats and ability to be able to do what xerneas can do aside arc fairy, and using arceus fairy = tremendous opporunity cost).
Some Xerneas sets require immense support (Geomancy for example), but others like LO + 4 attacks require next to none and simply just help wall break for a team. There's a similar deal with Cleric Xerneas and Choice Scarf Xerneas, the former provides aromatherapy for a team and it CAN beat Gengar while the latter is perhaps one of the best revenge killers in the game, being able to stop dragons, mewtwo's, Yveltals, and even clean lategame.
Arceus normal is weird, I can see how it can get S rank and how it could get A rank. Honestly I'm fine with either but anything lower is unacceptable.
Sorry if this post is massive and large and looks like a wall of text :(.