I would have said Hitmonchan but I finally decided to use one in Y as a gimmicky partner with elemental punches for elemental fangs Tyrantrum in double battles, I know it's retarded but I just liked the idea
anyway, pokemon I've always wanted to use and never had the chance to so far are Kingler, Sharpedo, Quagsire, the Electabuzz line and Aegislash because I couldn't fit one in my Y team
come to think of it, that's almost 6 pokemon