Assassin's Creed. Altair > All.
I don't like where it's going with the new ones though, I prefer to sneak around with the hidden blade and throwing knives rather than use all the gadgets they've got now.
Also the original Call of Duty, Medal of Honour, and Raven Shield at LAN parties. I don't really like playing multiplayer online, there's something strangely satisfying about being able to throw cold pizza at someone when you lose.
Supreme Commander is brilliant, every bit as good as the original Total Annihilation (Keep an eye out for another sequel
).