Oh, gosh, there's a lot; where do I even begin?
I suppose I'll start off with Mega-Charizard Y. Personally, what I like about this Pokémon the most would be its design. To me, it seems to be very swift, which it is. When it comes to battling, it's able to perform quite nicely; have been able to win a decent amount of online battles before thanks to it.
After this, is Mega-Blastoise. Its appearance is very bulk-looking, and its battling capabilities complement that pretty well. Similarly to above, thanks to this Pokémon, I've also been able to win a handful of battles. Aside from these two, few of my other favorites are Talonflame, Greninja, Delphox, Chesnaught, and Malamar (in no particular order).