Currently stands as:
Call of Duty 4: 8.16 million
Modern Warfare 2: 9.62 million
Halo 3: 10.83 million
Gears of War: 6.03 million
Gears of War 2: 5.58 million
Games in a series usually sell in similar numbers or grow rapidly as the fanbase grows. Seing as Gears 2 cant outsell Gears 1, Geras 3 wont come close. Although MW2 outstripped Cod4 easily its probably peaked so I doubt MW3 will outsell it. Halo 3, the current leader, came out at around the same time as Cod 4 when the Xbox 360 userbase was much smaller (around 11 million), so seeing how MW2 sold compared to Cod4, its likely Reach will do the same thing with Halo 3