>>18916416I'm far from good, but:
Mega Charizard, either one. X is physical, Y is special. Both are good.
Mega Mawile: Send in Mawile with Intimidate to lower your foe's attack, then mega-evolve to get huge power and destroy shit with STAB Play Rough. Jaws of Wife is pretty good.
Mega Gardevoir: Requires a transfered (or hacked) Gardevoir with Hyper Voice to really shine, but it's mega gets Pixilate which makes hyper voice hit like a fucking train.
Mega Lucario, Kangaskhan, and Blaziken: So broken Smogon banned them to ubers.