This is the best AC for me because I love some of the one-liners that Ezio comes out with while he's still young and cheeky, and it has his best and most obvious character arc. He starts out young and fresh-faced and gradually becomes much more stone-hearted with regards to the things he does towards the end - he kills Vieri di Pazzi by stabbing him repeatedly and shouting at him, but in the end he spares the life of Rodrigo Borgia, the architect of his sufferings (which turns out to be a bad call, but still). In Brotherhood and Revelations, he's already a stone-faced killer, and has the less obvious character arc of wisdom and fatigue in his work, which is still very good, but with perhaps not so much pathos.
The way of life is the posture of a tree; rigid trunk for structured and directed childhood, followed by open branches - life choices. Some branches take one higher than others, and some split many times. But never forget...Every branch bears a leaf.