For me, it's a tie between Reggie, Ash, Cynthia, Paul, and May. I missed the episode with Barry, so I don't really know anything about his character, other than he has a very similar personality to the game rival, and uses Paul's training method, to a lesser extent.
Reggie is the exact opposite of Paul. Also, assuming the badges we saw were in fact Reggie's, he's a better trainer than I thought. Of course, if you can use a Swalot, Bibarel, and Staraptor and then teach your brother's rival's Staravia Brave Bird, you're a pretty cool guy.
Ash I don't really need to explain. He's the face of the pokemon anime, along with Pikachu and now Piplup. He's a good trainer, but imho, he needs a lottle more development. For me, he's too flat.
Cynthia. 'Nuff said.
May I actually didn't like for a while. Now I think she was handled rather well, and she has great pokemon. She's a good character, and was the first of the main characters to compete in a contest. She's compassionate, and her pokemon are quite strong. She's also the first one to "replace" Misty. She was a good replacement.
Paul is ruthless, but is a great trainer. That's really all I can say.
Oh! And J is a favorite of mine. She's epic!