AC2 was the best of the series imo, i did like brotherhood and revelations too, but i just liked AC2 best.
dark sector is not a bad game either, quite a bit of multiplayer though if i remember but i never bothered with that only the single player.
a good game i can recommend if u haven't played it is Enslaved:OTTW.
cracking game imo.
http://hmv.com/hmvweb/displayProduct...;-1&sku=310058