Dawn.
I'm sorry, I felt the need to break the Misty lovefest.
Anywho ~ I strongly dislike May to begin with (I won't say hate, seeing as that's too strong of a word). So that leaves Misty and Dawn. Misty, while I liked her at some points, annoyed me. She still does. I don't care if she seemed more mellowed out in her two-part appearance in Hoenn, I still think she comes on far too strong. It goes past pride and independence and just becomes annoying.
I mean, getting angry enough to hit someone? For a small comment? No. It just isn't right, even if she is a tomboy. Do you see guys (normal, average guys) hitting someone whenever they said something just a toe over the line?
*Shrugs* Just my opinion on her. (Though, I will admit, I always liked her for pulling Brock by the ear. When she did it in Hoenn, I couldn't breathe through my laughter xD)
Dawn, on the other hand, has so much more potential to grow. Sure, May had to go even further, seeing as she didn't like Pokemon to begin with, but the point stands. May grew to love Pokemon, learned a few things, and that was about it from my viewpoint. I always got the impression she whined too much (even for a ten year old).
I just like Dawn. She seems so much more believable to me, and so much more real.