My favourite FPS, I must have played COD a thousand times. The maps are outstandinding, each an important real-life location for the allies in WWII, detailed and full to the brim with nazis.
The scenes are awesome and in places create a real sense of tension and fear. In one particular epic level, which requires the player to blow up oncoming german tanks with a set of badly damaged panzerfausts, the wait for reinforcements is genuinely tense and wearing (I'm obviously not real war material).
Each level is so different, requiring different skills. Some are machine gun based, some require a stealthy crawl into enemy territory, some say hello with a grenade in the face.
Anyone who like FPS should love Call of Duty; and there is no better platform to play it on than the PC. It also offers an epic multi-player/online link-up mode.