Mega Gallade became my favorite mega right when I saw it (the cape tho). Also, it's shiny has all my favorite colors on it @_@. It's been an ace for me lately.
Mega Charizard X has to be my second favorite, used it a lot in the battle resort to rack up tons of BP. It's power is just insane.
Side note: When I first saw Mega Slowbro, I was laughing for like five minutes straight just staring at it's poor face as it's getting devoured.