The three most recent editions of Call of Duty went one-two-three in the top 20 of Xbox Live's most-played titles, as measured by unique users connected to the service, reports Major Nelson. The only thing in the top 10 that wasn't a shooter or sports title was Skyrim, at #6.
Black Ops was #1; November release Modern Warfare 3 was #2 and 2009 release Modern Warfare 2 was #3, making Call of Duty the SEC West of video games. (That's a US sports joke, son, you're not supposed to get it.)
Halo: Reach was #4 and Battlefield 3 was #5. The full top 20:
1. Call of Duty: Black Ops 2. Call of Duty: Modern Warfare 3 3. Call of Duty: Modern Warfare 2 4. Halo: Reach 5. Battlefield 3 6. Skyrim 7. Gears of War 3 8. FIFA 12 9. FIFA Soccer 11 10. Madden NFL 12 11. GTA IV 12. NBA 2K11 13. Battlefield: Bad Co. 2 14. Call of Duty 4 15. Battlefield 3 Beta 16. Halo 3 17. Gears of War 2 18. Forza Motorsport 4 19. Red Dead Redemption 20. Call of Duty: World at War
In other categories, Pinball FX2 was the top Xbox Live Arcade game (measured by full versions purchased, in this case the base game, not all of its tables.) FortressCraft was the top indie game, Age of Empires Online was the top Games for Windows game, and Angry Birds was the top Windows Phone game. See the full rankings at the link.
Top Games of 2011 [Major Nelson]