With an excellent story, switching between US marines and British SAS, fighting in cities, snow, and countryside, using any number of (real) weapons) and one memorable mission in Chernobyl, CoD MW2 puts you in the driver’s seat (sometimes literally) of a great game. Anyway, CoD MW2 is an extremely polished shooter, and the single player campaign is every bit as enjoyable as the multiplayer maps. You may have noticed a trend here - Infinity Ward, the company behind the successes of these games, tend to improve their game sequels quite a bit.
With over 150,000 people playing the multiplayer game and nearly 70,000 people playing the single player every week, the only CoD game that has more players is Call of Duty, Black Ops II.
Call of Duty Modern Warfare 2 (which is the sequel to Call of Duty 4 Modern Warfare) is a classic first person shooter, and is still one of the most popular games in the Call of Duty franchise.