Call of Duty 4.
Modern Warfare 2 could've topped that easily, but IN MY OPINION, the customization/unlock system is rather flawed and the single player is INCREDIBLY short, lacking substance in exchange for flashy looks. Also, as a PC gamer, the removal of dedicated servers and mod support was a rather big disappointment, since certain aspects of multiplayer that are deemed "frustrating" could've been removed through mods.
COD4 was simply better in my opinion, the campaign was better, it had substance but it wasn't as flashy. The multiplayer was also better (PC wise), where you had more control of what you wanted in the gameplay instead of pure random stuff being shoved down your throat and there were mods to extend the life span of the game itself.