Mega Alakazam: I like that Mega Alakazam can bypass Mega Sableye and Chansey on stall teams with CM Dazzling Gleam and CM Focus Blast, and CM Shadow Ball can beat Slowbro and Jirachi, so it depends on the coverage in order for Mega Alakazam to beat stall teams pretty well. Inflikted said that it is forced to run Timid due to the new fast Mega Pokemon, limiting its power, and while this is true, I think it's a benefit that Mega Alakazam outruns many popular Mega Pokemon, so I'm think B+ is perfect for now.
Breloom: I agree with this moving to A- because its SD Orb set is very effective against the stall teams more common now than in XY, and Sash is still good against offense. I don't think the matchup issue vs playstyles is too hindering either.
Celebi: I definitely advocate Celebi for A-, and have for some time. I find that the roles its typing and movepool lend it to are more valuable in ORAS, and I can Celebi fitting quite nicely with the rest of the Pokemon in A- rank. Celebi checks many common Pokemon well, including Keldeo, Mega Lopunny, Mega Slowbro, Mega Swampert, Rotom-W, Excadrill, Mega Diancie, Manaphy, and completely walls TG + RD with Psychic > Ice Beam, the more common variation, Landorus lacking Knock Off, and Thundurus lacking Knock Off. The Choice Scarf set is a great Mega Lopunny check and is underrated in general, while Perish Song is a great boon on defensive sets as it is very helpful against the slow boosting sweepers which are becoming quite common. I think its extensive movepool is incredibly helpful as well.
Chansey: Same rank as Skarmory; it's just as passive, can't wall Greninja very well anymore, and metagame trends are unkind to it, like the rise of TG RD Manaphy and Encore Mega Lopunny. Quoting alexwolf: "it's bait for too much dangerous stuff, including most offensive / set up MEvos, such as Mega Metagross, Mega Lopunny, Refresh / Heal Bell DD Mega Altaria, Mega Gallade, Mega Sableye, and CroMegaBro. It also doesn't help at all that the best special attackers in OU can all get past Chansey, namely Gengar, Thundurus (NP + Taunt, NP + Focus Blast, Knock Off + Superpower), Keldeo, CM Mega Diancie, Greninja, Mega Gardevoir (Taunt or CM), and Landorus (CM or Knock Off sets)".
Dragonite: This was discussed when it moved here; nothing has changed since then.
Gyarados: A-, see
Halcyon's post which explain this nicely.
Hawlucha: Stays in B+.
Hippowdon: Hippowdon is great :) it walls heaps of stuff while being a bulkier, more resilient version of Tyranitar at setting up sand. It checks Excadrill, Talonflame, Thundurus (with a Rock-type move), Mega Pinsir, Mega Metagross, and Mega Lopunny. It provides SR, has reliable recovery, and isn't setup bait like other defensive Pokemon can be due to its decent Attack stat. It should move up to A-.
Kabutops: Should stay where it is, rain is still good, Kabutops hasn't changed much.
Kingdra: Should stay where is is, rain is still good, Kingdra hasn't changed much.
Kyurem-B: Stays in B+ rank, see alexwolf's post
here for a pretty in depth explanation.
Mew: Same as Dragonite, although a drop to B+ is feasible.
Raikou: There was something about Raikou that I didn't like in XY. I don't really know why, I think I was using CM Raikou during the Aegislash suspect test and was annoyed by its lack of power, but from recent experience, Raikou is far better than I remember. alexwolf said something about this in the council PM, that Raikou is the Electric-type with the least strain on teambuilding, as Mega Manectric uses a Mega slot, Thundurus is SR weak, and Magnezone is Choice-locked, and I agree, Raikou is far easier to slap on teams as a glue. Assault Vest and CM sets are still viable, but from what I've been using recently, Choice Specs is the best set, as it checks Mega Slowbro and switches into annoyingly common Pokemon like Klefki and Thundurus. I think it sits solidly in B+ rank.
Rhyperior: I used Rhyperior heavily near the end of XY but it isn't on the same level anymore and deserves to go down to B. It can't setup SR as easily thanks to Mega Sableye, it's setup bait for Mega Altaria, most of the new Mega Pokemon introduces threaten and/or force it, especially Mega Sceptile and Mega Metagross, and the rise in Pokemon like regular Gyarados gives it another physical attacker it struggles with, which is annoying when balanced teams try to use Rhyperior as one of their main checks to physical attackers.
Skarmory: Fine where it is. Pretty cool now that Skarmory walls a lot of physical attackers that have been popping up but still faces the issues of passiveness and one of the most popular physical attackers that it should be able to wall, Mega Metagross, it can't.
Starmie: Stays in B+
Suicune: Faces competition with CM Rest Mega Slowbro and CM Manaphy which have been more popular recently and the overpreparation for Mega Slowbro hinders Suicune as well as they have almost the same counters. B rank.
Mega Swampert: On one hand, Mega Swampert offers a lot to a rain team, including an Electric immunity and pretty good bulk, as well as Stealth Rock if you choose to not use Ferrothorn, but it's really held back by its poor Speed before Mega Evolving as it means that offensive teams can't just be ran through as well as you'd expect. It takes up a Mega slot which would be beneficial as Mega Manectric, Mega Sceptile, or Mega Heracross, all common Pokemon which offer a lot to a rain team and thus give Mega Swampert competition in its role. I think it's fine where it is, in B+ rank.
Tornadus-T: Not sure.
Victini: Not sure.
Alomomola: Mega Lopunny get setup Subs on this thing, so can Altaria, which isn't ideal. Mega Altaria fully makes this Pokemon a liability. It isn't really as useful on defensive teams anymore with its role being lent to Slowbro nowadays; it's passiveness hinders it.
Amoonguss: Maybe it's just me but I've always found Amoonguss a bitch to deal with because it doesn't die with Regenerator and its good bulk and had Spore to annoy teams. Now in ORAS, it can check Mega Altaria and Mega Diancie, while being able to Clear Smog stat boosters like Mega Slowbro and Suicune. B+ imo but I don't think I'd mind if it stayed in B.
Mega Beedrill: I haven't used this too much but from what I've heard, it's a B rank threat. It's very difficult for offense to deal with because of its high Speed and power, and someone mentioned that Protect is also a boon for it against the Choice Scarfers which are typically the only offensive Pokemon that can revenge it. Its frailty and vulnerability to priority, as well as common checks make me more inclined to vote B rank for now.
Mega Camerupt: I don't like Mega Camerupt. It's terribly slow, not even ok slow like Pangoro and Azumarill but ridiculously slow, slower than the common Pokemon on defensive teams, slower than literally anything else you'd expect to see on an offensive one. Every team has at least 3 Pokemon that do a truckload of damage to it including Greninja, Azumarill, Landorus-T, Latios, Latias, and Landorus, all not Pokemon you want to give free switches too, but unlike other offensive Pokemon troubled by different offensive Pokemon, Mega Camerupt is just so slow it can't do anything. Stall teams have no trouble with Mega Camerupt, they almost always have Chansey. Offensive teams can both kill it and even have a common switch-in, Latios or Latias, which are on a multitude of teams. I guess it's decent on TR teams but that's incredibly niche. B- or even C+, and putting it in C isn't even a stretch, imo.
Chesnaught: I think this should move up to B+. It's one of the better Mega Lopunny checks, but it also checks a slew of other common Pokemon like Bisharp, Landorus-T, and Mega Gyarados. Drain Punch is a pretty handy addition to its movepool too, and Spikes is more valuable in this metagame as well. It's a pretty nice option for stall teams
.
Crawdaunt: Not sure.
Mega Latios: Give me a couple of weeks on this one, it's intriguing.
Lucario: Countered by Mega Sableye on stall teams, not that fast and kinda difficult to setup too, I think it should move to B-.
Mandibuzz: Everyone hates on Mandibuzz :( I think it's still good in its current rank, and a good defensive Pokemon in general. It counters Mega Metagross, Mega Gallade, most Mega Charizard X, Landorus-T, Landorus, Garchomp, Mega Scizor, and most common Talonflame. It's one of the few Defog users that can Defog on the majority of Stealth Rock setters, and having access to Taunt and Knock Off give it utility against defensive teams.
Quagsire: Not sure, probably just fine in B though.
Omastar: Should stay where it is, rain is still good and Omastar hasn't changed much.
Scizor: I've seen regular CB and SD LO Scizor recently and they were pretty cool. It merits further testing but it should stay here for now.
Scolipede: I haven't seen one of these in ages so I couldn't really say anything, even though it's pretty obvious it isn't on the same level as the Pokemon in the rest of its rank. Offensive Scolipede struggles against the stall teams which are more common now, and with Landorus-T being everywhere and resisting both of its STAB moves, it isn't making a splash. SD Baton Pass and suicide lead sets seem to still be efficient though, so it could perhaps be in B- or C+.
Mega Tyranitar: This was moved here last update, it's fine.
Zapdos: Move down a rank, it's setup bait for Mega Altaria and can't check BirdSpam as well as it used to thanks to SD Talonflame being quite common. I guess it checks Mega Sceptile but that isn't exactly too difficult to wall. It has common switch ins for offensive teams and does nothing against stall barring getting off Defogs which admittedly is helpful but it's intrinsic to every other Defogger too.
this took way too long ugh