1 game that comes every year. Halo is a series we will get in some form every year. FPS, RTS, whatever, every two years at worst. Its a brand. Mario is the same, it comes out often and is successful.
Nintendo and MS have 1 really strong, top tier series, Sony used to have two, now they have zero, one at best if GT makes some crazy comeback but they have already told fans what they didn't want to hear about old standard cars returning.
MGS4 sold 6 million, not something just to throw away. GTA is a more pure example, but nonetheless both are very strong.