I have AC2, Brotherhood and Revelations. To give you a quick run through of what I liked about them here it is.
Liked AC2 for story and a game that didn't take 4 hours to complete.
Brotherhood for same reasons, and a better combat system
Revelations felt like they invested too much time in the multiplayer, I would rather have a game with a better story and made longer than a multiplayer thats had very little change to Brotherhood. Still liked the story and the change of scenery from Italy but it did feel like "hmm how can we wrap up Ezio's story really quickly? and move onto our next project?"
I wish they would realise they don't HAVE to keep making a new title EVERY year. Its ok Ubisoft, take a break work on it and come back.