Why is Xatu B? No, seriously. Do I even need to explain why the only Magic Bounce user in the tier that also has access to STAB Stored Power, Calm Mind, Dual Screens, Tailwind, Defog, U-turn, Thunder Wave, Trick, Foul Play, Wish, Haze, and Roost deserves to be ranked higher than B?
Because it fits the description of B-tier perfectly, that being it fills a defensive niche perfectly, that in itself being Magic Bounce. It is also part of the Combusken Pass to Xatu with Stored Power gimmick, yes, but that isn't really relevant as an individual Pokémon. Why doesn't it get passed to Musharna, Swoobat or even Girafarig? Xatu may be able to pull that gimmick off the best, but it is not something it does individually, and as such a gimmick involving another Pokémon so crucially tend to be considered in the main Pokémon's ranking, that being Combusken here.
Xatu is also outclassed in pretty much everything it does (this is off the top of my head and I'm somewhat new to this tier, so I'll be forgetting lots of Pokémon here lol):
Dual Screens - Uxie, Mesprit
Tailwind - Archeops, Pelipper, Swanna (the latter two are more personal preference, but Archeops outclasses Xatu 90% of the time.)
Defog - A lot of Pokémon, such as Pelipper, Altaria and Swellow
U-Turn - By this I assume you mean momentum grabber? Because Mesprit and especially Liepard do this much better.
Thunder Wave - Ferroseed, Granbull, Uxie, Mesprit
Trick - Rotoms, Mesprit, Uxie
Foul Play - This isn't even a niche lol. Even if it was Liepard gets STAB on it and is faster.
Wish - I can't think of a Pokémon with this right now, but honestly Healing Wish is better for momentum and healing, which Mesprit utilizes very well.
Haze - I'll give you this one. Xatu gets wrecked by most setup sweepers like Feraligatr anyway.
Roost - Again not a niche in its own right, so nothing "Roosts" better.
I'm not saying that Xatu is bad by any means, but it IS a Pokémon which belongs in B rank. A B rank Pokémon is classified as:
"Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outperformed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category."
Xatu fulfills a given offensive (Combusken pass) or defensive (Magic Bounce) niche. Xatu is sort of setup bait for some of the more popular sweepers, as it doesn't really do anything to them and is just KO'd in return lol. It is also partially outperformed by Mesprit and Uxie who are in A- rank, but is a good Pokémon anyway. This is why Xatu is in B-Rank and why, IMO, it should not be ranked any higher.
To get more in the given discussion, this is what my S and A rank would look like now (A rank would need to be changed a bit after some of them going up to S):
S
A+
A
A-
Changes: Combusken from A+ to S, Feraligatr from A+ to S, Liepard from A to A+, Mesprit from A- to A, Gurdurr from B+ to A-, Mismagius B+ to A-
Combusken - Does this really need reasoning? Combusken is being suspect tested rn ffs. We all know that it is the best sweeper, Baton Passer (if that's a role in its own right lol) and pretty much best Pokémon in the tier.
Feraligatr - So Feraligatr I saw mentioned before but I didn't really read the argument. I just want Feraligatr in S rank because it is probably the second-best Pokémon in the tier, and is definitely the most versatile, closely followed by Slurpuff. The thing which is really dangerous about is definitely that versatility. You don't know whether Feraligatr is SD or DD until after it sets up, by which time it's usually too late already without powerful priority like Sucker Punch from a Cacturne or Spiritomb. This thing is so dangerous it made Pokémon like Vileplume and Tangela more viable just because they could counter it well. I think that that is a quality an S-Rank should have.
Liepard - Best support Pokémon in the tier right now and shuts down almost all setup sweepers besides DD Feraligatr and Gurdurr with some good prediction I believe. It is also probably the best weather setter in the tier just because of its Prankster ability, as well as the ability to pick up momentum quickly unlike other weather setters which is more important than ever on weather teams. It also gets STAB Knock Off for some good power and other utility attacks such as Thunder Wave and of course Encore, as well as U-Turn which is the main way it picks up momentum. Plus, I think it can run some fairly good offensive sets. I haven't seen any work thus far, but my teams always seem to have counters to them lol.
Mesprit - Mesprit and Uxie are similar Pokémon, and they fit on different teams generally, but they are differentiable. For something like stall you should use Uxie over Mesprit. On VolTurn Mesprit is the better option. What I feel puts Mesprit in A rank is the fact that, like Feraligatr, you don't know what it is going to do before it uses it. Mesprit can work offensively or defensively with its base 95 in its physical and special stats, and it has the movepool to back this up with moves like Stealth Rock, Knock Off, U-Turn, Zen Headbutt, Psychic, Ice Beam, Energy Ball, Thunderbolt, Rain Dance, Sunny Day and its best option: Healing Wish. Healing Wish is another important thing which differentiates Mesprit from Uxie. It knocks out Mesprit and fully heals its replacement, which is fantastic on teams like HO or even bulky offense sometimes. Mesprit is one of those Pokémon who is a bit of a jack-of-all trades, but you can concentrate its moves into one role and unlike most it is actually good at its role most of the time. For example, if you're on a sun team, a set of Sunny Day, Knock Off, Healing Wish and Zen Headbutt with a Heat Rock would work. If you're using a HO team a set of Psychic, Ice Beam, Energy Ball and U-Turn could work with a Choice Scarf / Choice Specs / Life Orb.
Gurdurr - Gurdurr has been an underrated threat since last generation lol. Its bulk with Eviolite, as we all know, is absurd. It was so good that last generation it was viable when this tier was completely and utterly run by Psychic-Type Pokémon like Musharna and Gardevoir. This generation it only got better with the Knock Off boost making Psychic-Type Pokémon even less of a worry to it. It also holds the esteemed title of being the one Pokémon Liepard can't switch into and try and Encore to great success, just because risk vs reward tells it not to. If Liepard decides to switch anyway out of desperation, you can usually tell and OHKO with Mach Punch. Gurdurr has direct competition with Sawk; Sawk is a little more powerful, while Gurdurr is a lot bulkier. However, the aforementioned Mach Punch is a very important thing Gurdurr has over Sawk because it is a priority move. After a Bulk Up this move does insane amounts of damage for a priority attack, and while it may not have something like Recover, it tends to recover more than enough HP with Drain Punch. I'd say that Gurdurr fits the description for A-Rank better than B-Rank anyway: "Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently". Gurdurr does have one main flaw, and that is that it does get worn down quite easily throughout the match. It acts as the glue to a team, and while it is very durable it does lack recovery outside of Drain Punch, which can just not heal enough HP sometimes. This is why it should be only A-, but I think it is too good for B+.
Mismagius - This is probably going to be the most controversial suggestion on this list besides Feraligatr, but hear me out. Mismagius is very unpredictable; in special attacks alone it gets Shadow Ball, Energy Ball, Power Gem, Thunder(Bolt), Dazzling Gleam and Psychic. It gets two fantastic setup moves (Nasty Plot and Calm Mind) and a blistering base 105 speed. I've found a Choice Scarf set effective to nail stuff like Sneasel and Pyroar with Power Gem. It's probably the closest thing we have to a special set up sweeper in the tier right now, too, and it is good at what it does.
What changes would you make to this list?