Since, I personally, think that CoD 4, WoW and MW 2 are all worthless and I hate the very mention of their names, I would go with none. As, CoD 4 campaign was ok(I never really understood the whole "shock and awe" part. It took me ten minutes to die, and It was rather stupid in my opinion) and the Multiplayer(as evident more now) is full of arrogant and "no-skill"(I say no skill cause they all do the same thing, have the same perks, and go to the same spots) players that I really dislike. To save you and my self the rant(s). I'll just say this:
Get WaW if only for the Nazi zombies, which was the only fun thing it any of those games.