FraterPerdurabo: MW2 is very flexible in its approach to missions. Tell me another FPS that's better than that? Right now I can only think of Deus Ex and MAYBE (a big maybe) Project IGI.
KavazovAngel: Try playing Crysis or Far Cry. :)
MW2 is a movie compared to those.
Played Far Cry - the liberty there is an illusion and you have just as much in MW2. It doesn't matter how much empty space you have when it is merely that - empty space. Sure it makes the world look bigger but Far Cry is just as much on rails as CoD.
As for Operation Flashpoint, I agree, that's a very different kind of gameplay. If that's your cup of tea then well, go for it. I played it for a long time when it first came out, I doubt that I'd have the patience to do it again now.
That being said, some of the missions in MW2, especially Spec Ops leave you at a lot of liberty on how to approach etc.
edit: btw haven't played the other games on those two lists.