My favorite is Mega-Mawile.
My least favorite is a tie between:
Mega-Blastoise: All he got was a chin spike and two cannons on his arms. It looks overdesigned.
Mega-Gengar: It doesn't look like a ghost anymore, and it looks stupid. Plus, Gengar doesn't need a Mega.
Mega-Kangaskhan: First of all it's child abuse. Second of all, Kangaskhan doesn't even evolve, she just gets her kid out of her pouch. Third of all, that's two pokemon in one, so you team can have seven pokemon instead of six.
Mega-Mewtwo X: Mewtwo should never be anything but psychic.
Mega-Mewtwo Y: It just looks stupid. The original Mewtwo looks much much better.
Mega-Absol: They ruined Absol by giving him that ugly bang and wings. The original Absol is much better.
Mega-Garchomp: That jaw is creepy.