MW2 for sure.
It is up to which you are more a fan of. Since halo reach is newer i would guess that it is more popular at the moment
halo is a bunch of s*** it sucks 4 life mw3 is the best game EVER.
Halo 3 and Call of Duty: Modern Warfare 2 are both good games. It depends on the player though. Halo 3 is mostly unrealistic but is played for fun. COD: Modern Warfare 2 Is more realistic but sometimes people tend to become serious because COD involves more shooting than Halo 3. - Henry
Modern Warfare if you are only asking about video games, but anime is a most popular subject when it is expanded beyond the video game media
Shooting type games are more popular than sporting type games. Games such as Black Ops, Modern Warfare, and Halo are among the most popular shooting games whereas games such as Madden NFL or Kinect Sports aren't as popular.
Modern Warfare 2
Yes, plus more.
A good place to go is GameSpot.com (Not GameStop) and look at their highest rated xbox 360 games. That tends to be a good guide. A couple of the really popular games are Halo, Gears of War, Call of Duty: Modern Warfare, Assassin's Creed, and Grand Theft Auto 4, but there are a LOT more than just those.dont forget modern warfare 2
Each new Call of Duty: Modern Warfare installment has been successful in becoming even more popular than the preceding installment. Modern Warfare 2 sold approximately 4.7 million copies in both the UK and the US in preliminary sales. To date, copies for Modern Warfare 2 have sold at 117,000 for PS3, and 61,000 for Xbox 360.
Halo 3 is more popular then any Call Of Duty In fact, Halo 3 is the best selling FPS of all-time :\
Halo Reach has been the best seller but now MW3 is now popular than Halo Reach