Halo 3 should be the number one Xbox game. Put aside if people think it is overhyped or not, it is Microsoft's flagship title and managed to keep down CoD4, CoD: WaW, Gears 2, GTAIV, etc for the majority of the time. Halo 3 is also the top selling game on the Xbox 360 with over 10 million copies sold (which is insane) and it is one of the few games that really is "Only On Xbox". Number 6 is ok I suppose, but a little bit of a slap in the face in my opinion.
MW2 is great and all, but it really doesn't have that connection and history that Halo 3 does with the 360. I love both games though.