Koga for me! Middle-aged ninja FTW. Heck, all the Kanto gym leaders are awesome in their own rights.
Jasmine's pretty sweet, too. I bet most people challenging her would be all, "Aw, it's just a girl with an inferiority complex, what could possibly go JESUSCHRISTIT'SASTEELIXGETINTHECAR!"
Tate and Liza...d'aw, they're creepy psychic twins who have freaky celestial rocks-with-eyes for pokemon.
Fantina...I've said this a lot of times, but she's freaky in a cool way. And she has a Drifblim, one of my favorite pokemon.