Ehh... there's WAY too many moves for me to list just one. D:
I'm very fond of the elemental punches: Ice Punch, Fire Punch, and Thunderpunch.
And Hi Jump Kick would be another one of my favorites (this move absolutely SUCKS in the Mystery Dungeon games, though T_T), and I'm very glad its power was raised in B/W. Even though now that means... missing would probably result in a self-K.O. ><
And for some reason, I really like the special attack moves that can be found in a TM: Shadow Ball, Ice Beam, Thunderbolt and Flamethrower. :3 Oooh, oh~ And probably only TWO non-attacking moves that I LOVE would be Toxic and Thunder Wave. >:3