That's an absolute joke. The entire Xbox brand was built on Halo. There will never not be room for Halo.

This year: Halo Reach

Lifetime: Gran Turismo 5

My vote goes to Halo : Reach.

Halo Reach first week, but LF for SMG2.

Well, not the biggest of the Year, but I think NSMB Wii will remain the best selling game of the year, besides Wii Sports and until Reach comes.

legend of zelda games sell well but they aren't really geared towards casual audience so i doubt it will be near these 3 games. Then again it has wii in its name, so that will probably get a bunch of casuals to pick it up. I completely forgot about heart gold/soul silver. Its definitely gonna win for the year.
Game freaks really has made its fans crazy. I bet there are a million people who have or will buy all 5 pokemon games that came out on the ds that are the same exact thing. I mean i bought pokemon platinum and thats it, but i have quite a few friends who bought diamond or pearl, then platinum, and are now gonna buy one of the two new ones...
The only reason i wish they didn't sell so well is because why would game freaks want to actually work on improving their titles, if they can just repeatedly sell it making tiny changes or reusing old maps, and each copy will sell more then 99% of all games out there. They did this last time too, with ruby/sapphire, then emerald, and then firered and leafgreen and they are the three top selling games for the gba. That is just ridiculous. But hey it works.

It's gotta be Pokemon HG/SS surely? Everyone has been waiting for these games since the DS was first announced :P

Halo Reach, for the simple reason that it is Halo.

Natal Party... or whatever name that game will have that will be bundled with it..

Can I say New Super Mario Bros. Wii, or it just counts games from this year?

If so, I'm going to say SMG2 because it is launching much sooner than the other competitors.