I'm partial to Misty. I feel as though I grew up with her, so she's an instant favorite.
I've never really liked May, her personality was just a little annoying, but she's starting to become less annoying.
Dawn is just adorable! >.< She feels more like a little sister to Ash and Brock then a possible girlfriend.
So when it comes down to it, I think it's a tie between Misty and May for the role of Ash's girlfriend. They both seem to show a liking of him.