I tried call of duty 4: modern warfare at someone's behest. The Chernobyl levels were kinda cool. Said person was interested in the following two games, so I played those too, and I recall world at war being good.
I simply must stress that it is not a series I would be naturally interested in myself. I understand that it's some peoples reason for existence, though I'd imagine that it's basically a yearly sports title at this stage and one needs to be released. It's the devs I feel sorry for, they're stuck in the call of duty mines.