For me, it would have to be Lenora. I was still a noob when I first bought B/W, so I didn't really know what to do aside from spam SE moves. I did NOT go in there expecting a 140 Base Power move before calculating in STAB on the first turn, so yeah. Though now she's a breeze for me. But back then, man.
Bearkiller on PS and IRC. May also go by the name of Grima on IRC. I play League of Legends, so there's that. Also free Genesect plz. My LP.