Sabrina, ftw ><
Really, she's awesome. I don't watch the anime much, but I do read the Pokemon Adventures Manga (which is based of the games, not the anime) Sabrina's awesome in there (although a bit cruel). I saw Sabrina in the anime also, for four episodes, and she kicked *... (is this a profanity free web site?)
She's one of the only trainers that can KO Pokemon without even using her own Pokemon.
I also liked Erika for some reason (again, in the Manga, she rocks)
And I recently saw quite a few anime episodes and Dawn.....well, I don't hate her, but she's so annoying.