Halo 2
Units Sold: 6.61 million
Rank: 6
Publisher: Microsoft

It doesn't get much more hard-core than "Halo," or any more ridiculous. In this series of games you're an anonymous cyborg super-soldier battling hordes of aliens. So why is this Xbox game the only platform-exclusive title to make the list? Credit the game's ability to couple ranked online death matches with slick combat mechanics that had players switching fluidly between grenades, guns and melee attacks. The result is both silly and satisfying.