I think, despite what anyone else may think, that he'll end up with Callista. They seemed to get along very well, and they look so cute together.
That, or some young female trainer we have yet to meet.
And bwahaha, I counter Misty by saying she's fated to remain unmarried or find some random dude we haven't met yet. Or Dorien. She's matured beyond her childish one-sided crush on Ash.
And May? I think it's too early to tell. She's a little young to be truly in love.
Possibly Drew, but not Ash. Never Ash.
Firstly, age difference. Second, neither is a romantic like Misty was at times. And may I remind you that Ash is unlikely to take interest in girls until he's about twenty. It's obvious his mama didn't have that little chat with him about the birds and the bees.