If I got to choose one game from AC series, my most fav is AC: brotherhood. It had everything: parkour, amazing cities, charismatic heroes and much more. Ofc it wouldn't be so amazing without AC2. But mostly I love that every mission can be done with condition so brotherhood(and all next games) is replayable if you ignored at least some of conditions. Multiplayer wise is also brotherhood. AC4 is on second place for both single and multiplayer. After AC4 I first time thought that it would be amazing to play whole game as templar killing assassins.