Beats CoD4, Modern Warfare 2 and World at War.
Microsoft has revealed that Halo 3 was the most popular game on Xbox LIVE in 2009 based on connectivity to the service.
The Bungie FPS, released in September 2007, topped the LIVE chart ahead of Call of Duty 4, Modern Warfare 2, World at War and Gears of War 2. Next came Grand Theft Auto IV, Left 4 Dead, Halo 3: ODST, Fable II Episode 1 (a free download) and BioHazard 5.
You'll find the complete Top 20 over on majornelson.com.