Normally I would say GT5, since it has been 6 years in making and is the Noah's Ark for racing (Eurogamer); but since Reach is the LAST real Halo ever (Halo Legend 254 or watever don't count), so every Xbox owner might jump in the wagon to say good bye to Master Chief, Halo Reach might just outsell GT5. After Reach it would be all soccer mom's time on Xbox.