May, with Dawn a close second.
I've watched most of the anime, and I have to say that May had a really close bond with Ash that was just natural if you know what I mean. It wasn't just banter between them, but rather, mutual understanding and mentorship. I mean, after all, without Ash, May would have probably have never became a Pokemon Coordinator, and probably would have just gone sightseeing the Hoenn Region with her Torchic.
May really does look up to Ash, and in turn, he gives his best support as he can. For example, whenever she was down, Ash was always there to pick her up. For example, her first loss with Bulbasaur, or the time she thought she couldn't obtain her fifth ribbon. Ash was always there to pick her up.
The friendship of the two was got to the point where May even was imitating Ash's battling style, which was all-out swift offense, and practically no defense. This actually becomes her downfall in the Kanto Grand Festival in which Solidad defeated May with defensive maneuvers. This defeat practically shook May, as at one point she wouldn't even practice with Ash, due to realizing that being with Ash just wasn't good for her career, which eventually led to her departure.
The Terracotta Ribbon Ash and May share however shows that they still care for each other, and May has stated that her piece of the ribbon has saved her on many occasions within the Johto region, which I found to be touching.
TL;DR? May.
