After playing every Call of Duty, I'd have to say Call of Duty 4 is the best without a doubt. The list goes: CoD4, CoD2, CoD WaW, CoD, CoD BO, CoD MW2, CoD BR1, CoD 3.
1) Call of Duty 4
2) Call of Duty: Black Ops
3) Modern Warfare 2
Although Black Ops and Modern Warfare 2 are really close. They could pretty much be changed out between #2 and #3 for me. My issue with MW2 is the n00b tubes and cheating. For Black Ops, it's the spawns.
I would have to say Modern Warfare, as it was the first and most innovative and is one of the main reasons I went and bought and Xbox (actually the only reason, I bought the Halo 3 edition xbox and when they asked if i wanted the game was well, I 'politely' said no thanks I would like to play a real game, not some hop along game. I would say black ops is up there for the zombie maps and the hidden story line.
true at higher levels it gets kinda stupid how much ammo you have to pump into them, but the same was true for zombies as well, but atleast you could pack a punch your guns and make them better, not really much you can do about it in survival though you do get more options for support