hmmm difficult to vote. I kinda like them all..
ACI was just great, but became boring in the long run. So that one is not getting my vote.
ACII, brotherhood and revelations are all the same story about Enzio.
Then probably it's gonna be ACII, the story what happend to his father and brothers and how Enzio became the assassin he is today...And of course: NO MULTI PLAYER.
But then again, the game controls became better in brotherhood, but there was multiplayer
last but not least: I really like the settings in Revelations and the hookblade is fantastic. Bad thing (imo) is that there are more guards with rifles. (and there will be even more of those rifles in ACIII, and I don't want this game to turn into a 3rd degree shooter) The story of revelations is great as well...I haven't played the multiplayer yet and I'll doubt that I ever will.
Assassin's Creed does NOT need a Multiplayer
So I can't really choose between the last 3 games.