I don't know, MW2 was good, but there's something about CoD: The Big Red One (PS2) that really stands out. I love the bomber mission, and the missions actually have good objectives. Not just "go to this point, kill these enemies, and then run as fast as possible to this one". It had stuff like escorting medics around, blowing up mines. Also, on one mission, you had unlimited ammo for the Browning M1919. Nirvana. But that is only what I think. I haven't played the original, or MW.
Please do refer to the "Best games"-threads that seem to thrive here, instead of making a series specific thread. If you want to make it series specific, you can always do so on your post, giving a good review of your reasons, and perhaps showing several games for "best" in different categories.