Sales numbers are not always indicative of a game's quality.

[coolestguyever]

Duh. Sorry but you kind of stated the obvious. Killzone 2 is probably worthy of being in the top 25, or even top 10 shooters of all time but It will struggle to reach 2 million while GTA4, while it is a great game, easily breezed past 5 million on the PS3.