I would say Jimmy/Ethan, Clair, or Lance and here's why:
Jimmy
Jimmy, to me, is the more powerful version of Ash. His personality is better, he fits into Pokemon better, his friends are interesting, his pokemon are better in my opinion, the character himself is mature and isn't full of bull crap, and he's cool. I think they should replace Ash with Jimmy because Ash has gone on too long. He needs to retire.
Clair
Oh where do I begin. She, for one, specializes in my favourite type. She is beautiful, smart, tough, and friendly. It was a joy watching her battle Ash in Johto. She is epic, and not a total jerk like some of the other gym leaders were. She is unique in her spirituals. She is also self-confident which I find very attractive in females. I liked her a lot.
Lance
I swear he invented the word cool. Again, he specializes in my favourite type of pokemon. He is tough, his personality is colourful, he is smart/tactical. And, what more do I have to explain. If Lance and Clair weren't "cousins", i'd love to see them shipping together.
Sorry if this was a lame response. I tried, but they are my favourite characters! :)