I already said what I thought about Volcarona, Suicune, and Thundurus-T. I'll address the other nominations here.
Klefki: I agree with Klefki rising to B-. It has a really good Dual Screens + Spikes set, and so is a very useful lead for HO teams. It has flaws in its relatively poor bulk, being Taunt bait, and lacking Stealth Rock and Taunt, but it is able to make up for those flaws with priority screens. It also hates Defog, but when but on a team with Bisharp one shouldn't worry that much.
Cresselia: I think I also agree with Cresselia rising to C+. It really is stall's only way of defeating Calm Mind Landorus, which is a pretty big niche. It boasts in addition to the standard defensive set, it also has a Trick Room set and a Dual Screens set. It has reliable recovery (outside of rain and sand) and ample coverage. It's really solid for both HO and Stall, and I'd honestly push for B-. But I guess moving one step at a time would be better.
Gardevoir: It has a terrible speed tier and terrible physical defense. These are two major flaws that prevent it from being a huge threat against offensive teams. However, once its checks and counters on offensive teams, it will kill most everything. Its main perk is being a phenomenal stall killer; and it really is. With its ideal four moves, it beats Chansey, Skarmory, Venusaur, Amoonguss, Heatran, Quagsire, Alomomola, and Slowbro, all of which are common on stall. It's too slow and frail to be an A ranked threat, but that doesn't take away from how threatening a Poke it is. It belongs in B+
Medicham: Suffers similarly from what Gardevoir does. Its physical bulk is slightly better, at the cost of much worse special bulk and a much worse defensive typing. It provides no defensive utility to a team, and is a momentum killer against teams with Talonflame + Aegislash, which are obviously very common. It, like Gardevoir, is a phenomenal wall breaker, and should be ranked as such. It also beats Chansey, Skarmory, Venusaur, Amoonguss, Heatran, Quagsire, and Alomomola on stall. It's really hard to stop when against defensive teams, but against offensive teams it's more of a hit-and-run Pokemon that rarely gets a chance to hit. It belongs in B+.
Cofagrigus: I'm really unsure about this. I've never been up against one and I've never used it. Theoretically, it seems like a good defensive Pokemon with Mummy and a good offensive Pokemon with Nasty Plot and Trick Room, but I'm unsure. I'd give it D, but I don't play TR, so idk if its niche there is big enough.
Reuniclus: I'm also unsure. It hits very hard, and it's Trick Room set seems really cool. It has reliable recovery and an ability that negates Life Orb, weather, hazard, and status damage, good bulk, and insane power. I'd rank it D, but I don't play TR, so idk its niche is big enough.
Mew: Yes please. It's such a good Pokemon now, and B+ is where it belongs. TauntWisp is as good as always, and it has reliable recovery, hazard removal, and Knock Off it can also add to a set. Its main problem is that it can't use Taunt, Defog, Knock Off, Seismic Toss, and Psychic all on the same set (Soft-boiled/roost and Will-o-wisp are mandatory), but its ability to take hits and break stat is nearly unmatched. I'd rather use it than Gengar on my teams, but that's just me.
Alomomola: Such an annoying Pokemon to face. It's basically a physically defensive Chansey, but with smaller Wishes, no Heal Bell, and a better defensive typing. It's Taunt bait, but so is Chansey, and it's set-up bait, but so is Chansey. Alomomola heals as it heals a teammate, and because of this is very useful on stall teams. I'd agree to raising it up to B-.
Mega Aerodactyl: This is long overdue. It performs well against offense with its amazing Speed and surprising power when Adamant, and also against stall when running Taunt. It has so much coverage (Stone Edge, Earthquake, Aerial Ace, Fire Fang, Ice Fang, Iron Head, and Crunch) and a lot of utility (Roost, Defog, Stealth Rock, Taunt) so that you can tailor it to what you need on a team. B rank suits it the best.
Mega Absol: This change I disagree with. Mega Absol is very underrated in my opinion. It has a great speed stat, Attack stat, and Special Attack stat, which allow it to cripple many things with Knock Off, beat faster Pokemon with Sucker Punch, and cripple physically defensive Pokemon with a powerful Ice Beam or Fire Blast. It also boasts the ability to OHKO Tyranitar with Superpower, which is very useful for a Dark-type. Its main draw comes from being immune to Thunder Wave and Will-o-wisp and deflecting Stealth Rock, which is extremely useful for an offensive team. However, it has to watch out for its terrible bulk. If Rotom-W decides to Hydro Pump instead of using Will-o-Wisp, or if Hippowdon uses Earthquake instead of Stealth Rock, Absol is in for a world of pain. But, when played well, Absol's combination of mixed offense, great coverage, strong priority, and Magic Bounce make it deserve to stay B-.
Also, I have some proposals.
Noivern: This was brought up by someone before the recent update, so I'll bring it up again. Noivern is useful for its immense Speed, which allows it to act as the only Dragon-type that can revenge kill Latios and Latios. Noivern also lacks a Pursuit and Sucker Punch weakness, so it isn't a liability against teams with Bisharp, Tyranitar, or Scizor. Additionally, it has the ever useful move Flamethrower, which is stronger than Latios' Hidden Power Fire (despite the 33 Special Attack) and allows Noivern to deal with Bisharp and Ferrothorn, two Pokemon that give Latios nightmares. It lacks Defog and has a SR weakness, but it does come with other benefits. It has utility in Roost, Taunt, and U-turn, which allow it to support its team, as well as Focus Blast so that it can hit Tyranitar for decent damage. Its main draw is its speed, which allows it to revenge kill everything except for Mega Manectric, Mega Alakazam, Mega Aerodactyl, and Scarfers. It's overall a niche and effective Pokemon (
here's its analysis by the way), and I think it deserves a ranking again. I think it belongs anywhere between C and D, but I'm not sure.
Azelf: I think Azelf deserves B-. It has a nice suicide lead set, a Rain Dance set, a Nasty Plot set, and a Dual Screens set, all of which are useful on HO teams. The rain set in particular is nice to use on teams that used to use Deoxys-S. It has U-Turn, so it's able to switch to Rain sweepers instead of just dying, something Deoxys-S which it could've done. Overall its versatility and effectiveness make me think it deserves a higher rank.
Diancie: It's pretty mediocre, but isn't as bad as some people say. It has Stealth Rock, Trick Room, Dual Screens, and Explosion, all of which make it an interesting lead. It also hits decently hard with its two STAB moves. It checks Mega Charizard-X without EQ or Will-o-wisp, which is most (outside of stall teams). It can also run a Rindo Berry to check Charizard-Y (at this point I'm just kinda theorymonning). It's Calm Mind set is also interesting, but it dislikes that it has two physical weaknesses to watch out for while boosting, while Clefable only has one and Suicune has none. I'm not sure of its ideal ranking, but I think C- would be a good place to start discussion.