Since we're not counting Wii Play and Wii Sports I would have to say Super Mario Galaxy is going to take this one. I think GTAIV is going to get very close, but SMG has so many things going for it and nothing against it.
It isn't an M rated game, it's super accessible by hardcore and casual alike, he's the iconic video game character, a good majority of the people that already own a Wii will want this game, just about anyone that buys a Wii after it's released will buy the game, and Mario games have stupid long legs.
As for the Other contenders, why I don't think they will sell the most:
GTAIV: while this game is very popular and has very long legs, it's also rated M and very controlversial. Yes, people under 17 are going to get this game but M rated games do have a harder time reaching mass markets. Still, since GTAIV is going to be on multiple systems and it is such a huge seller, I think it's going to be really close to SMG.
Halo 3: Halo 3 has sold insane number and will continue to sell decently through the holiday season but the game won't continue to sell strongly pass the holiday season. It's going to keep being played like crazy until Halo 4 or a better multiplayer game comes out, but it just won't keep selling 3 million games per month.
Super Smash Bros Brawl: I know people love this game and are waiting for it (some even more than SMG) but it just doesn't appeal to everyone like SMG does and it won't sell as well. It's still going to sell A LOT but it won't be the high seller.
Wii Fit: This is another that is going to sell really well but just not appeal to everyone like SMG or have the kind of mass following that Halo 3 or GTAIV has.
FFXIII: These games always do well but SE actually lost people with FFXII and the fact that the game is the thirteenth in the series is impressive but it's also intimidating. I know everyone here knows that each FF game isn't really connected to the others, but not everyone knows that. People see FFXIII and think that since they haven't played the other XII they might be missing out on something important and won't bother. Yet another game that will do well but won't be close to anything else on this list.
