Thinking further on it Prez, I agree with you but it's impossible to qualitatively define the top first person shooter, so sales measurements are really the only worthwhile metric.
No, because by that logic a cheap family car would be better than a Lamborghini.
The simple answer is that there is no way to objectively prove which game is best. But given the sales figures and review scores I think it's safe to say that MW2 certainly isn't a bad game. Overrated? Almost certainly.