Activision recently stated it was COD4, 7 million sold through by GFK, Charttrack, and NPD. But I started thinking they didn't really include Japan.
So what was it? Here is what I assume are the main contenders:
Wii Play?
Wii Sports? (personally I do not think pack ins should count as sold games at all, so personally I would eliminate this)
Mario Galaxy?
COD4?
Halo 3?
A DS game?
Guitar Hero?
Also noted, Activisions COD4 boast likely includes PC sales..