Out of the ones I use (which isn't even half of them) I'd say Heracross. It just got nice stats everywhere and a good ability to make use of its moves.
Ampharos can also be devastating in a trick room, but its ability is really meh. (Unless hitting a mega Sceptile with Thunder is that important to you...)
I'm really fond of Mega Lucario, Scizor, Gallade, and Gardevoir even though they're popular.
I really like the design for Ampharos, but I can't manage to use him, no matter what. His typing is only... decent. ; ;
Heracross, Pinsir, Beedrill seemed like a wasted mega-slot in my opinion, but they're powerful.