They're all pretty terrible console ports. I wouldn't know any of the buttons in ac2 if I hadn't played through ac1 first, because in ac2 they decided to use icons to represent keystrokes as opposed to, say, actual keys.
I think you should buy all of them, smart people like me bought all 4 plus the dlc for $50 during the summer sale. In any case, I've been playing through them all. Currently at the end of the second one, it's a very enjoyable game but having not played brotherhood yet I couldn't tell you which one to get. If you literally can only get one I would guess that you should get brotherhood because of the multiplayer