I like the combat from the 3rd one a lot more. It was fluid and fast and the enemies weren't standing around waiting for their turn to die (like AC 1-Revelations). I've only played the online which is really fun, haven't got around to the single player.

Personally, I think they had a great story if they kept it to just Templars vs Assassins. The alien thing is too much. Although I like how they made our history fit with their story.