All the games will have their ups and downs. I'll most likely be buying all of them though. I've had MAG preordered and playing the beta since before MW2, and I can say after maxing out a few guys. As long as Zipper eventually adds more maps than the beta. It will be amazing. After you know what's going on, MAG was an extremely fun gameplay experience. And no, you don't HAVE to know the strategies behind MAG to enjoy it. Anyone can pick it up and play it like a normal FPS to just shoot people.
MW2, is well, MW2. It's fun, close quarters, fast paced. For the most part. Everyone knows about it so I won't bother.
BFBC2 looks like the winner in my book. Battlefield's always been great games. Ever since 1942, my first online shooter. BF was sick. 1943 is even super fun for being a game just thrown together for 15 bucks. The tanks and planes have always been balanced and the gameplay was never abused. It's rare to see anyone being a little b*tch and playing cheap (to put it nicely) in battlefield games. Everyone chooses their class, and plays accordingly. Hopefully, they'll have a good clan system in this one...