In many ways AC2 was better, but I didn't find it more enjoyable. The big thing for me was the setting. Italy during the Renaissance just didn't seem all that compelling - artistic, scientific, and architectural breakthroughs aside. The Crusades were much more intriguing to me, and seemed more apropos fit for such a concept. Plus, I'm not a fan of playboy-ish characters anyway, so Ezio didn't strike as interesting a chord with me as Altair.
Ultimately, I'm a bigger fan of beginnings than anything else (The Empire Strikes Back being an exception). Everything was fresh and innovative and mysterious. I also thought AC2, for all its gameplay additions and revamps, lost a noticeable amount of presentation value (Lucy's mouth got huge).