Treecko. No one said anything about "the character MUST be a human". I wonder if I should correct myself and say Grovyle, though. I guess that depends on where you're following the series.
If in any case we must talk about the human characters, then either May or Max. Not really sure why, possibly because we haven't seen them for several consecutive years. Also, putting a nerdish character like Max in as a main character is what drew me into AG in the first place. As for May, I believe it's just because she looks better than Brendon in the games. She also has a nice character about her.