Mega Mawile, Mega Charizard X/Y, and Mega Gyarados.
Ive always been a fan of mawile, even going as far as bringing mine to the hall of fame in my first playthrough of ruby! The mega evo is just what it needed. Nothing more invigorating than pulling off 1-2 intimidates before mega evolving. I love both mega charizards aesthetics(altho i lean more towards charizard x) and both of their abilities. New weather inducers are always welcome in my book! Lastly, mega gyarados was a perfect choice for a mega. Not being able to have any egg moves sucks, but now it gets a new dual type and becomes more bulky. I wish i could ride one irl :/