If I've never played Call of Duty, where should I start? If you simply wish to gain a feel of what the multiplayer is like, go with Modern Warfare 2. The leap in mechanics and features from MW1 to MW2 was a very large one, and MW2 would be the closest you'd get to what the gaming mechanics of MW3 are like.
Call of Duty 4: Modern Warfare and its sequel Modern Warfare 2 are not only some of the best Call of Duty games ever, but also some of the best and most important games of all time. While Modern Warfare set the gold standard for what these first-person shooters could be, Modern Warfare pushed it to an even higher peak.