DBZ fan's comment really breaks down why I thought Misty was the best.
Her fiery personality, dynamic with the group, intelligence, resourcefulness, and motherly tendencies (everything from her kindness, encouragement and even when she sometimes nags Ash and the others) makes her one of the most multi- layered characters I've seen in the anime.
A close second is May. Although Pokemon Contests annoyed me to bits and seemed unnecessary, I'm willing to overlook that in May's case because she went through one of the best developed character arcs I've seen in the anime. Contrary to what we see of her in the beginning, by the end of the Hoenn arc, she found a sense of purpose and independence and when it came to Max, she reminded me of Misty a lot. She really grew as a character (which is more than I can say for Reboot Ketchum.....err....I mean Ash).
I don't know why but everything about Dawn annoyed the hell out of me to the point where the Sinnoh arc took me the longest to watch. Iris, in the anime, is only slightly less annoying and seems to come off as incompetent.