It depends which side of physical or special you're looking for.
I found a lot of success in XY OU with Mega Scizor. I used this set:
Scizor (M) @ Scizorite
EVs: 216 HP / 96 Atk / 140 SpD / 56 Spe
- Bullet Punch
- Knock Off
- Swords Dance
It gave Scizor very good coverage and bulk, as well as a form of recovery in case it racked up damage, and then swept with +6 Bullet Punch. It was a marvelous sight to behold.
Mega Gardevoir is typically used as a stallbreaker.
I'd say something like a set of this would work for it:
Gardevoir (F) @ Gardevoirite
EVs: 24 Def / 252 Spe / 232 SAtk
- Hyper Voice
- Focus Blast / Will-O-Wisp
The bread and butter of this set are Taunt and Hyper Voice, which rip through most stall teams with ease, preventing them from doing much in return to you. Pair with a cleric, and you can keep Gardevoir around for a lot longer.