Halo isnt exactly a first class stake either though. It is like a McDonalds Hamburger served in a better plate at a restaurant and some special sauce instead of ketchup.I agree. McDonald sell more burgers than just about anyone in the world but I doubt anyone here would call them some of the best burgers ever produced.
But, GTA, Halo and MW2 aren't McDonald's hamburgers and the reason those titles show such high level of sales is due the ability of those series as a whole to produce quality titles over time.