For me, so far it's Absol, Mewtwo and garchomp. I like all of their mega form, but absol got a bit feminine :/ mewtwo is better the way it is, and Garchomp is one of the best pokemon of all time. It didn't need a new form IMO. Just look at regular garchomp in the new trailer! It's awesome!
Any pokemon you feel the same for, and if you don't get them, you won't megaevolve them?
Specter of shadows, ruler of the darkened moon, uncontrollable ~ Darkrai