MGS4 is't a true system seller... I suppose it was a minor system seller though.
A system seller first of all nearly always has to be the first on the system.
Wii Sports is arguable the biggest one of all time now, Mario Bros on the NES was back then. GTA is a huge one, Gran Turismo is pretty big, although Mario Kart is bigger (MK64 managed to outsell the PS1 GTs, and MK:DD had an immense attach ratio for the lowly GameCube)
Wii Fit is a seller too, as well as the obvious and many DS titles (which is why it is doing so incredibly) those being Mario Kart again, Mario Bros again, Nintendogs, Brain Training, the mighty Pokemon... and on DS at least, Animal Crossing.
Halo is a system seller too, I would say up there with Mario Kart.
Final Fantasy, Dragon Quest and now Monster Hunter are system sellers in Japan (in the same way that GTA/Halo are only system sellers in the West)
ANd what you said at the end about Halo is wrong.... Halo 3 was a big system seller, so while another Halo on the X360 will not make anywhere near the system selling impact, that doesn't stop Halo as a franchise being a system seller (as a theory, Halo 4 coming out on the next MS console would be a system seller)
