Damn, so many Black Ops II fans here... where are all the oldies at? My favorite Call of Duty game is a tie between the CoD 4 and MW2. Both were so good, each in their own aspects and the games that I've played so, so many hours of.
Call of Duty 4 was so good, it still is but I sadly lost my game key. I loved playing with my clan in a mode called Promod. We did ESL ranked games or scrimmage versus random teams. It was always challenging because there were many good players. I especially liked the CoD4 maps, like Strike and Crossfire.
Then MW2 - damn, who didn't enjoy getting a nuke then using it? it was too much fun. I always played it with friends, very rarely alone. I just liked everything about it, the maps, the controls, how the guns feel... all of the new perks that made the game feel fresh and so on.