I never would have thought a sandbox game could top maybe 2 of the best multiplayer games of all time, let alone 2 weeks in a row. GTA IV is what everyone is playing whether it be online or off.  Gears of War is still on the list despite being almost a year and a half old.  Dragonball Z also makes an appearance. Here is the rest of the online activity courtesy of majornelson.

Xbox 360 Top Live Titles (based on UU’s)
1 GTA IV
2 Call of Duty 4
3 Halo 3
4 Guitar Hero III
5 Gears of War
6 Rock Band
7 Tom Clancy’s Rainbow Six Vegas 2
8 GRID Demo
9 DBZ: BURST LIMIT
10 FIFA 08