Location.

No other game series came close to delivering the amazing sense of unique locales. From the parapits of the Middle-east, to Italy, to the American Revolution -- NO other series even attempted to match the sense of scale and beauty in AC's worlds. There are other parts of the series that need to be fixed or are sub-par, but damn... I really love how each time I boot up a game from this series, I feel like I'm not only learning a little more about history, but taking part in it.