I've always identified the most with Ax.
I'm not stranded away from home, and I'm not a genius. I don't know, I guess it's just parts of his personality that really speak to me. He's always awkward (Andalite or human-morph), but he tries. He does his best, even though he's often only guessing at what that is. He's submersed in new experiences, and he learns from them. What's better is that he doesn't always do a good job putting his research into action, or else he completely misinterprets something, which is sadly isn't unusual in my own life. I'm just glad that I don't quote These Messages. Despite his mistakes, Ax notices details in the world that few other people do -- although this, as well as his sense of aesthetics, is what supposedly makes him so "alien". Personally, I found it made him more familiar. Whoops.
People criticise Ax for being a follower. Even in the later books, the authors use the same type of language to describe his dedication to Jake's leadership as they do for voluntary Controllers! I could make a whole huge argument defending his character, but it essentially boils down to one point: I don't always agree with the decisions the characters make, but when I put myself in Ax's position, I rarely see myself doing anything different.
Ax is awkward and lonely, arrogant and intelligent, funny and loyal. I admire his strengths, and I see a reflection of myself in his weaknesses.