Halo 3 is a great game but it's just outclassed by Bioshock and SMG. If you've noticed, it's even dropped drastically in popularity since release, again with the TF2 release, and more than likely again with CoD4. The main thing that will keep Halo 3 from being GotY is that it's a pretty Halo 2 which was a pretty Halo.
I know you're just going to come back with SMG is just a pretty Mario64, and in a sense it is, but it's also more. Watch the GT Review and it gives you a good idea why SMG is special. Halo 3 didn't rejuvinate a genre, Halo 3 didn't make old gameplay feel completely new, and Halo 3 didn't really do much new to differentiate the game from the previous ones. Super Mario Galaxy does all those things and I'm fairly confident that it will keep up the high scores (with of course the occasional 85-90 score).
