World At War may very well be the best Call of Duty around. It basically is the same game as Call of Duty 4 but set in World War II. The game has a better single player campaign which consists of a much darker story, darker and bloodier graphics, and had language. It is the most realistic war game around.
This game isnt for everyone though. The guns are old fashioned and so are the setting. If oyu want a modern game with modern guns than Call 4 is your best bet. But World at War's multiplayer is nearly identical.
As an added bonus, there is a Nazi Zombie mode which puts 4...