I dont have managed it to play call of duty under linux
But I have heard about that they want money in the future to play cod.
Neverwinter Nights - The best available RPG for Linux. but at the moment I play Spring/BA/CA the most. Awesome RTS, really one of the best out there.
Civilization III, IV, V, VI and Freeciv.
quick and simple - endless sky
long but simple - Civ V
long and I can't accomplish a single thing that I want- EU 4