Have to go with Tobias. Then Ax, Rachel, Marco, Jake, and surprise surprise, Cassie is last.
Unlike most of the other responses I've seen here, my favorite character has nothing to do with who is most like me. In fact, if I were an Animorph, I think I'd actually be most like Cassie! But she's still last on my list, because she made so many stupid moral decisions where she put one life ahead of the fate of the world.
The World > one life <--This equation always holds true, no matter how much you think that 'all life is sacred.'
Mostly, I just like Tobias because he has a real depth of character, and because angst is fuuun.