For me it is Altair. However It annoys me when people say that Altair has no personality so Ezio is better. Altair does have a personality and for me it is better handled than in 2. It is more subtle. He has been trained from childhood so is now cut throat and heartless. However when his mentor punishes him for being like this he begins to lose faith. Finally when his leader betrays him he loses faith and his world shattered. He learns humility and the ability to admit to his mistakes and feels compassion for others. Ezio also has a character arc but his seems more arbitrary and secondary. For the reason that it is very poorly carried through the story if at all. Whilst Altair's arc comes through after each assassination as he has more and more doubts about his cause. Finally I find that altair is more awesome as he is pretty much immune to the piece of Eden due to his force of will and the strength of his mind.