My favorites are definitely Mega Gallade and Mega Blaziken. I always choose Torchic as my starter and now that Blaziken has a Mega Evolution in it's remake... it's just amazing!
The Mega Evolution of Venusaur is the most poor Mega Evolution I can think of. I am not entirely sure if it was also introduced in ORAS, though.
Both Mega Venusaur and Mega Blaziken were introduced in X & Y. Blaziken's was actually obtained via an event that happened for the first few months of the game's launch where players were able to download a Torchic holding the Blazikenite (Blaziken's Mega Stone) via Mystery Gift.