Considering the Favorite XY Mega Evolution thread is inactive, I'll post my favorites for both XY and ORAS:
XY:
My two favorite Megas in these games are Mega Mawile and Mega Aggron. The former managed to go from a forgotten Steel-type Pokemon to something so great, the Mega itself was banned to Ubers and for good reasons because of it's amazing new type combination and ability for instance. Mega Aggron could have better recovery but I just love it's ridiculous bulk (Defense-wise) and both have some pretty sweet designs imo. I also like Mega Medicham, Mega Garchomp, Mega Banette, Mega Blaziken, Mega Aerodactyl, and Mega Blastoise for similar reasons as well.
ORAS:
Mega Beedrill and Mega Rayquaza take the #1 spot for me. I personally find their designs simply awesome and slick. Especially Mega Beedrill considering it can punch holes into a lot of Pokemon thanks to U-Turn (having to use up a moveslot for Protect is kinda disappointing but that doesn't bother me too much. And we all know how ridiculous Mega Rayquaza is. Slowbro, Steelix, Swampert, Sableye, Primal Kyogre, Audino and Diancie are also up there too (really wish ORAS had at least one more Unova Mega though).