Battlefield 3.
Why? The last two games of Call of Duty - Call of Duty Modern Warfare 2 and Call of Duty Black Ops - is basically the same. Different Single Player, i'll give you that, but when it comes to Online game-play which I favor over Single Player, it's just another Call of Duty game. Same process. Play, level, prestige, repeat. Then when you're at the highest prestige possible, what else is there to do? There are people with like five hundred thousand CoD points that they can't use because they've completed the game as much as they can. Now it's just an online game.
Also I won't even go into the supposed 'teamwork' that's meant to happen in a team mode of an FPS. It's like Free for All but with less players to kill.

Now when we compare the two, there is a clear difference. For one, there is actually communication. Sure, there's the occasional 'I'm the best ever, your mum's a slag' talk but when I play it, there is always some form of teamwork communication. We play as a team, we let the team know what we're doing to help the team, and all out it's a much funner experience than just whipping out a sniper and one-shooting everything in sight. Yes, the only mode is Conquest (correct me if I'm wrong) but I'd rather have one mode than multiple modes that I can't keep up with. There are different skills that need to be put in to each mode on Call of Duty. In Domination, you need to be stealthy. In Free-For-All, you need to be quick. In Team Death-match, you need to know how to support your team (though there isn't much teamwork I see anyway..). In Battlefield, there is one mode. Whilst it may get repetitive playing the same mode over and over again, it's easy to learn, easy to keep up with, and it's fun. A game isn't meant to be all 'im'a snipe 'yo ass!'. A game explains itself. It's a game. It's not a competition. If there are game competitions, you can be all like 'Im'a rape you, sir', but in a game, I'm looking for fun and teamwork.

Yes, Call of Duty and Battlefield are immensely successful chains but the main reason why I prefer Battlefield over Call of Duty is because of the team-work. It gets on my nerves when you join a Team Death-Match on Call of Duty, wanting to get some team-work working, and all you hear is pre-teen kids shouting 'OMG WTF HAX' down the microphone. How their parents even let them play the game is beyond me, and when I do try to get some team-work in action, all I hear is 'Aw, shuddup Hawkzy 'ya ___in' noob'. With Battlefield, I only have to join a squad and I hear instant 'You head down, check for snipers, I got your back. Take the flag, got armor support near you'.
Solo play in a team mode is a bastard.

So yeah, Battlefield 3 for me. I understand that some of you will have varied opinions about this, so I'm probably guaranteed a groan.. :\