Call of Duty 4 is by far the best Call of Duty game. That game was so revolutionary and formed the way First Person Shooters are made today. It was also the first game from the CoD series that really went viral.
A close second place is Modern Warfare 2, and then third goes to Call of Duty 2. In case you noticed, all of those were the Call of Duty games developed by (the old) Infinity Ward.
I personally played, I enjoyed World At War the most, although I enjoyed Black Ops as well. Reason for enjoying World At War, was some of the simplicity to it, there isn't a million different kill streaks. Same goes for Modern Warfare the first one (although didn't play that one).