Mega Sceptile: It has a great new unique typing. It's become even more Specially Powerful, and while its ability seems to be mostly a double gimmick, in singles it could create a Gyarados/M-Sceptile core that is only weak to Rotom-Frost and Tyranitar. With Intimidate, it would take a notch out of physical attackers, and electric moves would be virtually null and void. A good ice counter teammate would be important, just in case something comes that could threaten them. Plus, there this:
Serebii said:
Mega Sceptile can cut off a portion of its tail to fire it like a missile at an opponent.
Is it just me or is that awesome?
Mega Swampert: I don't know about him. Swift Swim seems... kind of pointless. On the other hand, it could get M-Heracross level attack, and it seems that under the rain he would be fast enough to sweep. It's major grass weakness isn't too bad, as grass isn't very common, so depending on the tier it starts in, it could easily end up like M-Mawile, or could be stuck in UU because of the grass. Maybe OU would pull up some grass for this guy. It's hard to tell. I think he would also be good in doubles, having a rain-bringing partner and itself having Wide Guard, Earthquake, and Rock Slide. Maybe Sap Sipper Goodra to partner?
Mega Slowbro: Does what Slowbro does, but better. No criticals is nice.
Mega Sableye: Can you say OP? I see this as a monster. Mega Evolving the first turn, it fires off a T-wave while still having the prankster boost. Now Sableye cannot be taunted, and rocks/spikes/whatever can't be used, as it gains Magic Bounce. Assuming it isn't banned, it would cause some mind games, even if it doesn't have the mega stone. It's like Aegislash starting in Blade forme, being unable to change from Shield Forme to Blade forme. No one would know when it's safe to set up. Additionally, it gets Taunt and Mean Look, which could make it a weak pseudo-M-Gengar.
Mega Sharpedo: Well, it looks good... and we don't know much about it. Strong Jaw seems to be pretty useless in comparison to it's ordinary abilities.
Mega Camerupt: Water is a problem, but Sheer Force? Nice! It can easily abuse it with Lava Plume, Heat Wave, Flamethrower, Iron Head, and Rock Slide.
Mega Altaria: Since weather seems like it's about to be more important, Cloud Nine could help. Pixilate + Hyper Voice is sweet, and it could also use Heal Bell.
Mega Salamence: I have mixed opinions on this one. While Aerilate + Body Slam could be interesting, a Defense boost is puzzling. I suppose it could be a paraphaser with Dragon Tail? It gets Defog and Tailwind too.
Mega Metagross: I was looking forward to this, but while it looks awesome, I simply don't know about it's usefulness. Tough Claws is nice for Ice Punch, Zen Headbutt, Steel moves, but I don't know how much use it is over all the others.
Mega Lopunny: Scrappy. Normal and Fighting STAB. This thing looks deadly. It's come a long way from the rabbit no one wanted. It used to have low stats, and a limited non-egg move movepool. But with Scrappy and Attack boosts, it can use any fighting move and any normal move on any pokemon.
Mega Gallade: While nothing is currently known about it, it looks awesome. Not to be included for anything yet...
Mega Audino: Do not use unless the point of your secret base is for free EXP.
Mega Diancie: This is interesting. The characteristics that make up Carbink and Diancie, the defenses, are actually reduced for more attack power. It does have Magic Bounce, so it could set up well, but it seems a waste to use your mega for stealth rocks.