Kanto: Venasaur, Blastoise, Charizard X&Y, Gengar, Aerodactly, Gyrados, Kanga, Mewtwo X&Y,
Johto: Ampharos,
Hoenn: Blaziken, Absol, Mawhile, Gardevoir,
Sinnoh: Lucario, Abomasnow, Garachomp
Unova:
So far Kanto has the most Mega Pokemon with eight. Although there are rumors that some have gotten Gavantula stones as well as one for Druddigon. I have a feeling each region with get 10 Mega Evolutions. Going by the trade marks, Jynx and Pinsir would leave Kanto with 10 (12 since Charizard and Mewtwo have two forms).