Before I go into my thoughts on rankings, I wanted to give some feedback to people saying that something isn't a counter because it can't switch in if it has x amount of prior damage: one of the core aspects of competitive battling is analysing your opponent's team, spotting threats to your own team and
keeping your checks / counters / win conditions healthy accordingly. Fair enough, your AV Azu may not be able to switch in on their non-GS Ninja anymore but that's your own fault for allowing prior damage to rack up. It was still a counter when the match begun, you just played it poorly. All decent players understand this...
Anyway, onto rankings:
Mega Alakazam | Stay at B+
Not a lot to say about Megazam if I'm honest. He's received an equal number of positives and negatives in the ORAS meta so far. On the one hand, I think speed creep has been a boost for him, as it makes him all the more relevant, being able to still outspeed the vast majority of the unboosted meta. However, I feel what made him most threatening in XY was his ability to simultaneously provide an answer to Offense (with speed, power and coverage) but also Stall (with Trace shenanigans, Taunt, CM Psyshock for blobs etc). Now, with the advent of Megalye and Megabro, Zam just can't pressure stall as much. Zam can't OHKO Mega Sableye with Dazzling Gleam, especially after a CM, and won't want to be taking its STAB on. Same goes for Bro, boosts just get in the way, and with these two threats Stall isn't nearly passive enough for Zam to take advantage of in most cases. Zam also has great utility vs Rain with Trace, but that goes without saying. He has his uses, but it's a double edged sword. Keep him where he is.
Breloom | Raise to A-
This has been discussed to death and there's little more to add. Grass is getting better in the meta (especially Spore / Sleep Powder), priority is better because of speed creep and many teams just aren't prepared enough for Loom.
Celebi | Raise to A-
I haven't actually used much Celebi but hot digitty is NastyPass dangerous. As aforementioned, grass is proving a useful and underprepared for typing, and in Celebi's case it's the sort of lack of preparation you really can't afford. I've been steamrolled by a surprise NastyPass more times than I care to admit. I've used a physical set recently on mid-high ladder as well with Seed Bomb, Sucker, ZHB and SD. A bit gimmicky, sure, but the typing and lack of preparation from opponents meant it worked surprisingly well. Scarf and support sets are also highly viable, as has been discussed, and Healing Wish is a thing iirc, but I have no experience with them really so I can't comment. All-in-all, just like Mew last gen in its ability to do a load of things really well. Bump it up IMO.
Chansey | Stay at B+, maybe lower to B
Literally so little to say. As passive as ever, but often the glue a lot of Stall teams need. Can still check Ninja fairly well in most cases, which is probably reason enough for it to stay at B (get suspected plz Gren). However, it's still often easily taken advantage of my offensive threats you can't afford to be awarding free turns. A tricky one, so I'm on the fence.
Dragonite | Lower to B
Heavy opportunity cost. Other things can do its job better in general, namely Mega Altaria as a DDer. Also suffers from the power and speed creep, and reliance on prediction vs new threats like Mega Diancie, Mega Metagross, Mega Sceptile, Mega Lopunny... The list goes on. In my mind, it has nothing extra going for it except gimmicky WP sets and, of course ESpeed. Don't get me wrong, it's a sick move that gives Dragonite great cleaning capabilities an a definite edge in many clutch situations, but that isn't enough to mitigate its shortcomings. Definite drop if you ask me. Poor Puff :'(
Gyarados | Raise to A- (Edited... didn't realise Megados was separate)
Even after editing this, I still think Gyara should move up. His typing is great in the meta and his insane bulk, which is exemplified by Intimidate, often makes him a good answer to new threats like Mega Lop, evolved Megagross, Megallade etc... He's also a popular choice from a synergy point of view atm, as he can make up 1/3 of a FWG core whilst simultaneously being part of an Intimidate core. Definitely something that's underprepared for - especially RestTalk sets.
Hawlucha | Stay in B+
Yeah, Hawlucha is a lot more viable now that Aegislash is gone, and, I would argue, can be one of the deadliest late game sweepers in the meta. However, it's still just a one time setup. It's just so reliable on getting the unburden boost and an SD up before it can be really threatening. There's a lot that walls it or just causes it problems in general, like Lando-T, Sylveon on stall and Thundy. Admittedly, Sub-Sitrus sets are pretty awesome, and can screw people pretty badly (especially Megalye switch-ins predicting HJK). So it has definite merits. But on the whole, it can be tricky to use. The support and removal it requires just don't make it a threat worthy of the A ranks IMO. But somebody feel free to prove me wrong.
Hippowdon | Lower to B
I never see Hippo anymore. It's okay as a wall in certain situations I guess but the power creep, and rise of strong grass types in the tier, make its job so much harder. It basically gives MegaScep a free sub unless it runs Ice Fang, and Scep behind a sub is such a slut to deal with. Mega Sableye and Mega Slowbro also make its life harder. The meta hasn't been kind to Hippo at all. I also think powerful wallbreakers like SD Diggersby and Crawdaunt are really good right now and Hippo is just a tad too passive to deal with them. Giving Diggersby a free SD is not a good idea. Like. Ever. And nothing wants to take a LO Adaptability Knock Off / Crabhammer. Sand also isn't as dominant as it was in late XY, and TTar is struggling for exactly this reason. Deffo move hippo down, it just can't handle the pressure as well as it used to, and struggles to win a spot on a team in favour of other useful support 'mons.
Kabutops | Stay in B+
Rain is really, really good - nobody is denying that. But Kaputops's effectiveness in the metagame is no different to how it was last gen. MegaPert is the real reason rain has improved. Kabutops actually struggles against many of the new additions, most notably MegaBro, and also doesn't appreciate the rise of priority. Celebi is also a bitch for it, as Stone Edge rarely does enough to a healthy one. So yeah, Kabutops shouldn't rise just because more people are playing Rain Offense. It doesn't do enough new to warrant a rise.
Kingdra | See above
Kyurem-B | Raise to A-
Super anti-meta, as everyone has been saying. Choice sets are pretty meh if you ask me, although Scarf has a decent enough surprise factor when played correctly, but they're just momentum killers a lot of the time. Sub-3 Attacks Roost-3 Attacks, and even AV, are what make this guy scary. Sub-3 Attacks is awesome against like every playstyle, bar hard Stall, with the right switch-ins. Roost allows it to maintain crazy pressure from start to finish. And AV allows it to check a load of Ninja variants. Even if it just U-Turns out, Kyube is hard as dick to switch in on. Definite candidate for a raise.
Will make some further nominations later on but my fingers are hurting a bit and I want to see some more riveting discussion :)