There are a few Megas that have yet to be ranked. I'm going to nominate them, seeing as how every Mega evolution should be good enough to be used in OU at least. None aren't viable choices.
Mega Aggron for B+/B
252+ Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 252+ Def Filter Mega Aggron: 225-265 (65.4 - 77%) -- guaranteed 2HKO
That calc is just there to illustrate Mega Aggron's bulk. That's the strongest Physical attack in the game that Mega Aggron just happens to be weak to. Mega Aggron doesn't even come close to dying. When you have a wall that doesn't die to any unboosted physical attack in the game, you know that Mega Aggron is definitely worth the Mega slot if you have the right teammates for it. Mega Aggron's physical bulk is so great that it can leave it completely uninvested and still take hits from a significant amount of physical attackers.
252 Atk Garchomp Earthquake vs. 252 HP / 0 Def Filter Mega Aggron: 117-139 (34 - 40.4%) -- guaranteed 3HKO
252+ Atk Choice Band Huge Power Azumarill Waterfall vs. 252 HP / 0 Def Mega Aggron: 114-135 (33.1 - 39.2%) -- 99.9% chance to 3HKO
252+ Atk Choice Band Talonflame Flare Blitz vs. 252 HP / 0 Def Filter Mega Aggron: 168-199 (48.8 - 57.8%) -- 96.1% chance to 2HKO
Most physical attackers would need boosts to break through it, but setting up on Mega Aggron isn't a wise choice as it has base 140 Attack, and a strong STAB Heavy Slam which hurts much of the lighter metagame. It also can phaze with Roar/Dragon Tail. However, having no recovery outside of Rest + Sleep Talk holds it back from being the perfect tank, as Rest-talk is a bit unreliable even though sleep mechanics have changed back to Gen IV mechanics. However, that flaw shouldn't hold back Mega Aggron from a B ranking as it isn't as bad a flaw as it would have been last generation.
Mega Banette for C+
Mega Banette is a bit of an oddball. It certainly isn't the first choice for your Mega slot, and for good reason. Mega Banette didn't really get the stat boosts it needed. It got an amazing ability in Prankster, and it has access to some of the best moves which benefit from this such as Destiny Bond, Will-o-wisp, and Thunder Wave. But instead of getting a much needed increase to its bulk, it got a rather redundant boost to its Attack. Sitting at 165, Mega Banette has one of the highest Attack stats available, but only has a few moves to abuse this with; Shadow Claw, Sucker Punch, and Shadow Sneak are the most significant. The Mega Evolution mechanics also hurt Banette the most, as Prankster isn't activated the turn that it Mega evolves. Banette is very strong and has useful tools such as Prankster Destiny Bond, but being a Mega where stronger ones are available holds it back. I don't think it should go higher than C+.
Mega Houndoom for B-
Mega Houndoom is powerful and fast, with 140 Special Attack and 115 Speed. Coupled with a STAB combination that few Pokemon resist, and boosting moves in Nasty Plot and Sunny Day (because of Solar Power) Mega Houndoom looks like a threatening special sweeper. However, Mega Houndoom competes with other Megas, especially Mega Charizard Y as a Fire-type special attacker. It's also quite frail, which means boosting is hard and sweeping isn't an easy task either with the large amount of priority in this metagame. One must not count Mega Houndoom as a threat though, because it's still extremely powerful when it does get a Nasty Plot boost and sun support.
Mega Ampharos for B-/C+
Mega Ampharos has an incredibly interesting Electric/Dragon typing, giving it great coverage with its STABs and many useful resistances. With great 90/105/110 bulk and a gigantic 165 Special Attack stat, Mega Ampharos looks like a solid Pokemon. It's very slow, sitting at 45 Speed, but this can work to its advantage by taking hits and then using a slow and powerful Volt Switch to bring a teammate in safely. However, one major thing hold Mega Ampharos back: absolutely no reliable recovery. This is a huge setback for a bulky pivot like Ampharos, as it becomes very easy to wear down. Strong neutral hits or supereffective attacks combined with hazards quickly bring Mega Ampharos down, making it an underwhelming choice as a Mega Pokemon.