Oh god, what have you done to yourself?! Asking a question like that is going to leave you drowning in the flood of suggestions that everyone (including myself) is going to send you! DX
Now that that's out of the way, since nobody has said anything about any of the mega forms, I'll be the first by suggesting Mega Charizard X!
I'm gonna have to go with my favorite Pokemon, Haunter!...Or at least I would if I had even the slightest idea on how you would do such a thing, so instead I'll go with my runner up of epicness that no one ever picks, Alakazam!
Now that that's out of the way, since nobody has said anything about any of the mega forms, I'll be the first by suggesting Mega Charizard X!