Straight from Major Nelson's blog, it's the Xbox Live Activity List! If you were to combine the lists below, Gears of War would be the most played Xbox Live title.
Xbox 360 Top Live Titles (UU's)
1 Gears of War
2 Call Of Duty 3
3 Tom Clancy's Rainbow Six Vegas
4 LOST PLANET OnlineDemo
5 Madden NFL 07
6 RoboBlitz
7 Call of Duty 2
8 Oblivion
9 Ghost Recon 3
10 Small Arms
Top Arcade Titles (Full Versions)
1 RoboBlitz
2 Small Arms
3 Hexic HD
4 UNO
5 Texas Hold 'em
6 geometry Wars Evolved
7 Ultimate Mortal Kombat 3
8 Contra
9 Street Fighter II' Hyper Fighting
10 LUMINES LIVE!
Original Xbox Top Live Games
1 Halo 2
2 Battlefield 2: MC
3 Madden NFL 07
4 Counter-Strike
5 Call of Duty 3
6 Star Wars: Battlfrnt 2
7 Forza Motorsport
8 Rainbow Six 3 BA
9 Ghost Recon 2
10 Call of Duty 2