It's a toss up between ACII and Brotherhood. Revelations isn't in the running because there is no horseback riding. I loved the storyline in both ACII and Brotherhood. Gameplay was amazing in both.
Overall I guess Brotherhood beats out ACII. I rather enjoyed being able to touch back on some of Ezio's memories from when he was a young warthog.
I'm going to have to agree with BSH 1975 about MP, it's definitely not needed in this kind of game. I wish they spent more time on single player for Revelations as the story seemed rather short.
Working on / Boosting:
AC Revelations, CoD: Black Ops, CoD: MW2, CoD: MW3, Forza 2, Forza 4, GTA IV, Halo Wars, Hardwood Spades, Madagascar Kartz, PES 2008US
06 Arcade + Mobile Games Complete
26 Retail Games Complete (mostly non DLC)
01 Retail Games Incomplete + maxed out
Most Recent Completion:
NBA 2K12 (+dlc) - 1250
Jetpack Joyride WP8 - 200
Milestones (From 2/11/09):
10,000 GS - 11/12/2009
20,000 GS - 8/7/2010
30,000 GS - 11/2/2011