get COD4 as it's the best one of the lot.
Couldn't agree more tbh.
I've got MW3 and BF3, and it depends on my mood as too what I want to play. If I want quick action with a lot of fragging then it's MW3. If I want more tactical game play where you can get a good squad together and have an epic 500 ticket round of conquest, then BF3 all the way.
But as above, the original COD4 is still the best one.
MW3 isn't that good because there's no dedicated servers, it picks hosts like consoles which I hate, and you get a lot of douchebags on there complaining about campers and calling them noobs. Half these kids have probably never played a game competitively before and if they had they would realise sometimes you have to camp to defend objectives and a good player knows how to outsmart the campers.
BF3 is for me a little frustrating because it's more tactical and you can benefit a hell of a lot if people do simple things like use squad perks and work together. But people tend not to do those two things, get greedy trying to get kills all the time because everyone wants to rank up and get XP a lot faster all because equipment upgrades take a fair amount of time to get. And that greed leads to lone rangers which take out some of the tactical pleasure of the game.