For me, probably Cryogonal. It has a great Special Defense and decent Special Attack, plus it has Levitate, but its movepool is terribly shallow, with a meager variety of special attacks to take advantage of its Special Attack stat, and its Defense stat is so poor it makes Alakazam look bulky.
I mean, it's a killer snowflake! Wouldn't it have been obvious to give Cryogonal moves like Water Pulse or Ominous Wind to take advantage of its Special Attack? Wouldn't Power Whip have been a good move against Rock-types considering it has icy tentacles coming out of its mouth? What about a good annoying move like Wrap? And why does it know Attract if it can't do anything with it?
I hope Gen VI finds some way to improve Cryogonal, be it with some good moves or an evolution, maybe even a new useful ability, because I think there really is some untapped potential to this Pokemon.