New Megas Tiering:
Mega Sceptile: I want to say UU here. It will be crazy fast, probably on par with MAero and MZam. Lightningrod gives it a good amount of switch-in opportunities against things like Rotom-H that like to spam Volt Switch. With the added Dragon Typing, some of it's shitty weaknesses are alleviated, leaving it weak to only Poison, Ice (x4), Dragon, Flying, and Fairy, which isn't great, but gaining a fire neutrality in such a fire-spam filled tier is always nice. It will probably have its uses in OU as arguably the best Rotom-W counter in the game, but I highly doubt it will be used enough to be in the OU tier.
Mega Swampert: My guess is UU/BL, with a solid use in OU on Rain teams. It's already in UU, so there's really no way a mega evolution would DROP him. UU is a tier where speed doesn't matter as much as some, power is appreciated, bulk is important, and Fire-types are everywhere. Because of this, I think MSwampert will fit right in. Depending on stat buffs, it might even hit BL.
Mega Slowbro: OU. With that massive defense, boosted special attack, and an abillity that blocks crits, this thing just screams Calm Mind. Be it a Crobro set that forgoes the more reliable Slack-Off for a way around status in RestTalk, or a more offensive set that runs something like a second attack or Trick Room, Mega-Slowbro will be a solid OU mon.
Mega Sableye: UU/OU. It's typing is great, Prankster works on the turn it mega-evolves, it gets a good boost to both defense and special attack, and once it mega-evolves it can only be statused through the likes of Scald. Also, it has reliable recovery, and two very spammable STABs. Sounds like an amazing Calm Minder to me. In my opinion, whether it's UU or OU will be based off how it measures up to Mega Slowbro, since both appear to be built for a CM sweeper.
Mega Sharpedo: OU. The only reason it can't be in OU now as an amazing cleaner is it's lack of power. With buffs to every stat and Strong Jaw, that STAB Crunch is gonna sting, Ice Fang is going to be great coverage, and STAB Aqua Jet means that it can clean through even Talonflame. Also, despite losing Speed Boost, it can just protect the turn before it mega-evolves, get to +1, then mega evolve and rip the opponent to shreds. It looks like it'll have solid uses in OU.
Mega Camerupt: UU. It will probably be amazing on Trick Room teams, and Sheer Force STAB Fire Blast/Earth Power is jizz-worthy, but it is gonna be so damn slow, have no recovery, and retain a crippling typing in Fire/Ground. With Keldeo, Azumarill, and Rotom-W all being great atm, I can't see Camerupt fitting in in the OU enviorment. That being said, since UU is chock-full of Fire-Types, being immune to burns and having Ground STAB to smack them about with will give it a decent niche.
Mega Altaria: UU. Fairy/Dragon isn't really as great a typing as people seem to think it is. It would have been nice if it had Play Rough because a bulky DD set with its STABs and Roost would have been solid, but sadly it doesn't get that. Honestly, I think it'll probably be just your average tank, nothing too special.
Mega Salamence: OU. This thing will be a monster, but with it's most notable buff seemingly going to defense, not Attack or Speed where it could use it most, I doubt it will go uber.
Mega Metagross: OU. It's typing got better with the Aegislash ban, it gets a huge speed buff to speed that it desperately needs, and smaller buffs to all it's other stats, which were already good to begin with. Also, Tough Claws gives it a Bullet Punch more powerful than that of Bulky SD Mega Scizor assuming it gets at least 20 points more in attack.
252+ Atk Tough Claws Metagross Bullet Punch vs. 252 HP / 0 Def Mew: 109-129 (26.9 - 31.9%) -- 34.2% chance to 4HKO after Leftovers recovery
44+ Atk Technician Mega Scizor Bullet Punch vs. 252 HP / 0 Def Mew: 105-124 (25.9 - 30.6%) -- 3.5% chance to 4HKO after Leftovers recovery
It also gets boosts to Meteor Mash and the elemental punches, like Miror said.
Mega Lopunny: RU. Typing isn't special, Stats are so low 100 more points wont help it THAT much (even if every single point went into Attack it would only hit about as hard as Kube). Only notable thing about it is Scrappy HJK.
Mega Gallade: UU. Abillity isn't useful, typing is nice, a speed buff is greatly appreciated, going to be entirely outclassed by Mega Medicham, so it wont see the light of day in OU.
Mega Audino: RU. Garbage abillity, typing is only decent, loses the only thing that made it usable (regnerator)
Mega Diancie: UU. Sure it's supposed to get super fast. Sure it's supposed to get big boosts to attack and special attack. But that doesn't fix it's problems. It cannot run both STABS off the same attack stat, meaning it has to go mixed. Its defenses get worse, making it even more prone to being picked off by Bullet Punch and Aqua Jet, which already cause problems because it's typing was terrible. There is no way this thing will be good in OU.
Those are my thoughts, anyway!