Will go through all of the possible changes on higher ranks right now. Will try to get straight to the point on all of them:
Doublade and
Meloetta - from
S to
A+ (the former's been brought up the most, but since dropping Meloetta is a possibility, I might as well go for it anyway)
Both have defined the metagame a lot, but as it's known by now, that also brings some drawbacks regarding the Pokemon's effectiveness. Doublade is extremely one-dimensional and is usually the easiest piece to wear down out of most defensive cores. However, its only set brings a lot of utility to the table due to the amount of Pokemon it checks and its ability to sweep unprepared teams. Meloetta is the opposite of Doublade, and the best term to describe her role in the metagame would be: "jack of all trades, master of none", due to its ability to punch holes with its Choice Specs set, serve as a glue for stall teams with its access to Perish Song and Heal Bell, be the centerpiece of teams thanks to both its Calm Mind and Relic Song sets and so on. However, even with that sheer versatility by its side, it's not rocket science to figure out what set Meloetta is using by taking a look at the team it's in.
If we consider how much these Pokemon shape the metagame (Doublade keeps Cobalion, Virizion, Slurpuff and many other Pokemon from running rampant, while Meloetta helped making Spiritomb as popular as it is), it's safe to keep both of them in S. However, since that's not what this thread is about (or so I think), I'd push both of them down to A+, especially Doublade.
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Delphox - from A+ to A or A-
Has a gigantic niche as an offensive Fire-type that is worth using over Moltres under some occasions. However, the existence of the later in the metagame hurt it immensely, as everyone is already over prepared against Moltres, and most of the counters for the later also fare well against Delphox. However, since Delphox still has some niches in the metagame (Switcheroo, Calm Mind, and can even form some nifty offensive cores with Moltres), it's completely fine to keep it in the A ranks, but A+ is really stretching it out.
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Hitmonlee - From A+ to A
Really fell out of flavor. A Pokemon commonly seen on Hyper Offense that can't really stop Gligar from Defogging is sort of prejudicial for the playstyle's effectiveness, although it's true that it provides utility through Rapid Spin and Knock Off. Its main STAB move feels really awkward too, since it's impossible to spam safely until very late in the game (and Hitmonlee doesn't really have the Speed to clean up either). A rank it is.
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Mega Abomasnow - from A to A+ or S
Yes, you have all of my support for this one. This is
the Pokemon for high level RU play. It manhandles the typical balanced team, and the metagame's never been kinder for the frosty tree in general (sorry for giving it these nicknames, but it's just so adorable). It also has the advantage of having two (or possibly more, but no one's gone further than two right now) viable sets, negating any issues in regards to being one-dimensional. Besides doing well against balance, it also has amazing match ups against both offensive (thanks to its deceivingly good bulk and priority Ice Shard) and stall teams (ability to go mixed or Swords Dance. Both sets do well against that archetype).
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Amoonguss and
Druddigon - From A to A-
Two really good Pokemon that fulfill specific niches that, in my opinion, just aren't worth ranking so high. Amoonguss is always seen doing the same thing, and while it's good at that, it fails to check enough threats to keep it worth ranking in A (for the record, it gets demolished by some of the most common Pokemon, such as Moltres, Meloetta, and Abomasnow). Druddigon is versatile, but it's actually quite easy to find yourself thinking about its 4 MSS. The Druddigon set I've found the most success with was a hybrid of the Stealth Rock setter and the fully defensive set, with Stealth Rock / Dragon Tail / Toxic / Glare. While two status moves seems counterproductive at first, Toxic is the only way Druddigon has of getting past Defog Gligar, but sometimes the ability to paralyze something is required (against faster hit and run Pokemon for example), so Glare is kept as well. Being one of the few viable Dragon-types in RU also helps it too, but it just doesn't have the traits that Dragon-types like to have (ability to go mixed, set up moves, etc).
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Escavalier - from A to A-
Good Pokemon. However, the amount of support it requires to work properly (Wish, hazard removal, etc), mean that it just doesn't cut out as an A ranked Pokemon. It's also total set up bait for Fletchinder, which I'll get to right now:
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Fletchinder - from A- to A
100% dependent on Defog support. However, it brings unparalleled amounts of utility to a team, thanks to its strong priority, recovery, Will-O-Wisp, Swords Dance, decent defensive typing and so on. It wrecks a ridiculous amount of teams, and can set up for free on the aforementioned Escavalier, Gligar and some others, while also wearing down its common checks with Will-O-Wisp.
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Spiritomb - from A- to A
Bouffalant has managed to cover this one well enough. It's a strong utility Pokemon that supports the most commonly used Pokemon flawlessly, while also checking a myriad of threads on its own. It's also worth mentioning that it has another more under-appreciated set, CroTomb, which can serve as a win condition for stall teams, an even more solid Reuniclus check (although it needs some investment to 2HKO it with Dark Pulse assuming equal amount of boosts).
I'll go over the lower rank changes some other time, but for now, I think that discussion should be focusing on these.