My favorite Mega Evolution in Pokémon X & Y is Mega Charizard X. I know that it is a common answer, but Charizard is my favorite Pokémon and I think Mega Charizard X is a well designed Mega Evolution.

My favorite has to be Beyblade Slowbro it is by far my most preferred over all Mega Evolution's, it has slowly grown on me since the reveal to the point where I absolutely love it.

As odd as it sounds, I haven't seen enough of them to really have a favorite. I didn't really use a Mega in X, I occasionally used Mega Metagross and Sceptile in AlphaSapphire. Used a bit of Altaria, Steelix, Manectric and Slowbro competitively.

I kind of like Mega Altaria 'cause of its type and Slowbro 'cause of its defense and special attack, but that's only like 9% of the Megas that currently exist.

I like Absol, Manectric, and Lucario quite a bit. A few of them are fairly creepy, though.

I like most of the steel type ones like Lucario, Metagross, Mawile, and Scizor

I like them all i guess.

Mega Altaria, because how would you not want to cuddle that thing.