Here's a list of my top faves of the ORAS Megas:
Not only is it cute, but a powerful attacker who can also take a hit. Pixilate really made this thing work wonders both in-game and competitively.
Another cute Mega, but one with a lot of speed and power along with near-unresisted STAB coverage that not even Ghost-types are safe from thanks to Scrappy. Also, it made me love the Buneary line all over again!
Such a tank, and gotta love the buffs to his Attack as well. I actually had a shot wrecking a team under the rain on Showdown and I had the fun of using Mega Swampert in-game as well, given the Mudkip line is my fave Hoenn starter.
It may look derpy, but I really love this Mega because it's a tank and Shell Armor makes it able to abuse Calm Mind without being in fear of a critical hit. Trust me, it gets frightening if you let it set up too much.
I mainly liked this one for looks, and it is a fairly good defensive tanker. Healer may not be the best ability for it in my opinion, but I definitely can't say no to that Fairy-type.