You're really an idiot. Just because MW2 has stunning graphics and was tremendously hyped doesn't mean it's going to stand the test of time.
For the greatest FPS ever, I'm going to have to go with Quake. It pretty much was the father of all online FPS games. If a game is compared to CS:S, then it can be traced back to Quake and then Doom, the original FPS.
Source engine was the son of GoldSrc, which was pieced together from Quake's engine code.