Swampert is literally a battleship with cannons at every part of its impenetrable hull. It can really give Rain a boost, and is great in that it can't be beaten by Rotom-W. Definitely OU, probably around mid rank since Rain isn't among the best strategies.
Sceptile is something between Mega Manectric and Mega Gardevoir, and I think it will fundamentally function better than both. Paper thin defenses and 4x weakness to Ice as well as Flying priority don't help, but otherwise it has potential. Probably low OU.
Sableye has a niche in being a potent wall that cannot be crippled by the common Taunt+Will-O'-Wisp combo. Loss of Prankster hurts though. UU probably.
Altaria is too... average. It's like a boosted Kingdra. Good typing, about the same number in every stat, although it's slower, bulkier and stronger. However, 110 in offenses isn't exactly sky-high. I could see it being a more defensive Pokemon, since it can use its great typing to its best. Low-mid OU, I guess.
Gallade rocks, and 165 Attack along with 110 Speed and not too shabby bulk make it a very potent threat. Having its butt kicked by Talonflame and Pinsir isn't nice, but it can still outspeed Pinsir and slower variants of Talonflame with Shadow Sneak and overall hit like a truck. Mid-high OU.
Audino is just completely outclassed by Sableye and Altaria as a defensive threat... it could function as a cleric, but I think it's really a waste of your Mega slot if you do that. BL2 is quite possible.
Sharpedo is strong, and fast. Speed Boost in vanilla also is helpful. I'd say mid OU, mostly because it's just so fragile.
Slowbro is an amazing physical defender, and Calm Mind can really make it extremely threatening. High OU.
Steelix is somewhat similar, but it doesn't have a way to boost its Special Defense. 95 isn't all that bad though. However, as a Sand team Mega, it is probably outclassed by Garchomp. BL, or UU.
Pidgeot has a number of drawbacks that can hold it back, namely low defenses, lack of coverage and No Guard being a double-edged sword. It will most probably be UU.
Glalie could function as a mixed attacker, but pure Ice is never good, and it's probably outclassed by other mixed attackers, especially considering it takes up your Mega slot. UU.
Diancie has a fear of Steel, and bad coverage. Apart from that, though, it's downright amazing. The two former may hold it back from being a top OU threat, but I can definitely see it in mid OU.
Metagross is also great, while not having Diancie's problems. High OU.
Hoopa is still slow, and its typing is bad, however its offenses are amazing and its special bulk is great. Maybe low OU, or BL. UU is probably too weak for it.
Camerupt is meh; Sheer Force is great with 145 Special Attack, but it can't hold a Life Orb, has horrible Speed, a horrible Water weakness and weakness to Ground. It does resist Electric, but still. UU probably.
Lopunny is really nice. Strong, fast, perfect coverage. Low or mid OU.
Salamence is gonna kill everything, ressurect them with its godlike powers, then kill everything again. Top of OU, if it doesn't end up getting the boot to Ubers.
Beedrill has amazing Attack in combination with Adaptability and crazy Speed. However, it lacks the moves to abuse them to the most, has horrible defenses, is weak to SR and Spikes... in other words, it can die to a Fighting move. BL, maybe low OU.
