Brotherhood will always be my favorite. Feeling like you actually played an important role in the assassin order by sending out your assassins to numerous locations instead of being the pawn that's getting sent, and the great movement options from the horses to being able to shove people out of you way (unity). The only downside would be the story, which every AC game nowadays seems to follow. Assassin falls in love with girl. Assassin has to leave girl. Assassin reunites with girl. Girl dies. Assassin avenges girl. Even the real world counterpart with Damon followed the exact same plot, except over 4 games.