I have to say that CO2 was by far and away the best. Granted the MP version was not that interactive, but the maps were cool and the sniping was BAD ASS! The newer games got too overdone, I like to keep things simple.
Call of duty 4 was good because all of the skilled snipers that it had, and the maps were great. Call of duty: WaW was pretty good, but got ruined by all of the MP40 users. Modern Warfare 2 is my favorite because of the killstreaks, guns and perks. It does annoy me sometimes when I get in a lobby full of campers using Scar with heartbeat sensors.