AC I Was a great game, but the gameplay was very lacking... but still very enjoyable. But Altair, the only problem was that he wasn't anybody. You have no background info, no knowledge on how he joined the Assassins... nothing. All we know is that he's badass.
Very badass.
I loved AC II even more, but I agree, the tone of the story could have been more dark, I enjoyed it. You get to see you Ezio was, and who he became, and who he turned assassin, unlike Altair who was just... so overall Ezio had a lot more Character Development.
Though I would've like the Uncle to betray him or something... :/ Disappointment there.