Amos, Helmut, Latro, Nano, Aiyano. I won't pick just one; I can't really rank them from best to worst.
If I'm being objective, I'd say Latro, because I played the game from start to finish, and there was enough time for both development and completion. Amos was ruined for me, Helmut was cut short, and I wasn't given a chance to develop him, although I really liked how things ended with the female characters – I honestly don't know.