Obviously Ninetales, Arcanine, Rapidash, Flareon, Moltres, Typhlosion, Magcargo, Entei, Ho-Oh, Torkoal, Infernape, Magmortar, Heatran, Victini, Emboar, Simisear, Darmanitan, Chandelure, Heatmor, Volcarona, Reshiram, Delphox, Talonflame, Pyroar, and Volcanion (IK I ripped of Xiri's joke, chastise me if you feel it necessary)
Funny to think that thus far there hasn't been a single mega Gen 6 Pokemon despite the mechanic coming out in that gen.
Furfrou, Diggersby, Pyoar, Noivern, Trevenant, Avalugg and Pangoro all could use a Mega evolution.
Although I won't be satisfied until there are enough mega Pokemon so that you won't know which member of a persons team is a mega.