From what I have seen so far, Mega-most-things are basically approaching legendary in stats. I've seen a lot of people battling with nearly pure Mega teams, though it seems single-evolution Pokemon don't get quite as strong even when Mega-evolved.
Beyond that, I haven't really seen any Pokemon which stood out to me like "Whoa that is really overpowered!"
Xerneas and Yveltal will be Uber, Geomancy Xerneas sounds insane and Yveltal's Oblivion Wing/Aura-boosted Dark Pulse is basically unresisted, couple that with solid bulk and I can see both being solid Uber.
I can see Mega Lucario/Blaziken going Uber, Blaziken is a given honestly even with weather nerf. Mega Lucario hits so much harder than it's LO counterpart due to Adaptability, 2HKOing Skarmory unboosted with CC after SR is insane.
I really can't see Azumarill making a jump to ubers. It's good, maybe good enough for OU, but it's completely walled by a lot of pokemon. All because Heatran walls most fairies doesn't make it uber. I think you're over-hyping the new type. Heatran is still easily walled and most of the old counters are still very viable. It's like saying Scizor should be banned because it's really good against grass types. Ground still murders Heatran and it's not like the magma pokemon is going to break a world speed record anytime soon. The biggest jump I could see is Scizor going to Ubers. However, I find that very unlikely, but Scizor is going to be top OU for sure. Better start carrying a counter for it.I'm pretty sure that with all the type changes there should be some more Ghost, Poison and Fire types in the uber metagame, with Gengar being the most likely, as well as less dark, fighting, and dragon types. Maybe Azumarill could have a shot at ubers now that it learns Play Rough along with Aqua Tail, along with the awesome ability in Huge Power. Water/Fairy I suspect should have great coverage together. Heatran could be a permanent Uber too because of the strong type advantage over fairies.
Having no LO recoil is nice however and CC is still a lot more powerful on Lucario-M than Lucario LO (okay it's only like 15% more but it could be noteworthy but can't be bothered running calcs), powered up BP is also nice. However yes I actually changed my mind, I forgot Lucario-M has 4MSSWell to be fair, Skarm is a horrible switch in to most fighting types. Also regular Lucario 2HKO's Skarm with SR too with SR present with unbosted cc.
Mega Luke still isnt beating Gliscor, Salamence etc and even if it uses Ice Punch it has things like Gyarados, Jellicent, Defensive Dragalge, Aegislash, Tentacruel etc to deal with and regardless if what it runs it isnt beating Mega Venusaur because of Thick Fat. It also has alot of misc checks/counters like Qwilfish (who is amazing tbh, also walls mega Hera and Blaziken) and even Nidoqueen. Also Megacross is a fantastic check to it. I mean heck it can also be revenge killed with Bullet Punch resistant faster Pokemon and those are pretty abundant. It has alot of checks and counters tbh.
Blaziken i agree with because power up punch, speed boost and drought make it really not have many counters/checks at all.
I can see Aegislash being Uber. However, it can be walled. Avalugg, Volcarona, and Rotom-W do a decent job of walling it. Up to a certain point of course. I've done quite a few battles myself and just about everyone and their dog is using it. King's shield is one heck of an annoying version of protect and to be honest, I would love to see this pokemon get banned.From what I have seen from most of the 6th gen Youtube battles so far (Shofu primarily.), I would have to say Aegislash if anything due to not only the borderline godly twin 150 base stats but being able to shuffle those stats around thanks to Stance Change along with Swords Dance + Shadow Sneak to set up at a moment's notice honestly makes me a tad uneasy just thinking about it right now. Maybe Excadrill can revenge kill it but what I am wondering atm is if it can swtich into a +2 Sacred Sword and still stand to put this sword back into its stone. (Apologies for the horrid joke there.)
Another Pokémon that I'm expecting to be made uber is Mega Tyranitar. Its base Attack stat equals to one sixty-four, which is spontaneous. With one Crunch, it could take down even the most defensive Pokémon in an instant, such as Skarmory.