ACIII, cross platform and each iteration seems to increase the sales of the franchise, even the WiiU might get a port since straight porting from PS360 will be quite easy.

OH YEAH! forgot about FIFA

Black & White 2 should see some rather nice sales. If it does as well as Black & White in Japan (which is likely) and releases around the same time (September), it'll do 5 million just there alone. In the first 10 weeks - outside of the holiday season - Black & White did 5.2 million in Americas and Europe.

In other words, we're looking at 10-12 million for Black & White 2 provided it doesn't launch too late, and that it launches in Europe. Single-platform, no game is going to beat that, I doubt even Call of Duty will. Although the X360 version obviously has a chance to.

 Multi-platform, I'm no expert, as I simply don't know enough about release schedules.

Fifa 13 was mentioned earlier, and it's definitely up there. The '12 version did nearly 9 million in 2011.

If there's a Just Dance game ths year, that'll have a chance to. Just Dance 3 also did nearly 9 million in 2011.

Assassin's Creed is likely to be completely chanceless. The best selling entry to the series - Assassin's Creed II - sold under 6 million in its first year. Jumping up to over 10 million is very unlikely.

Of released games, the best selling one is likely to be Mario Kart 3DS at 6-7.5 million or so.

Halo4 by miles, surely? Doesn't Halo sell about 4 million day/week 1?

Halo 3 did 6.8 million in its first year, and Halo:Reach did 7.1 milllion. You're most likely going to have to go over 10 million to be the best seller, so it's unlikely that it's going to be Halo 4. 

Fifa, AC or Pokemon

Seems like several people are with me on the Mario thing! ^_^

to be fair on my opening post i did mention a 2d mario on 3ds =P. But ya if one does end up being released, it will surely sell massively, especially if it gets a a holiday bundle like super mario 3dland did.

Just Dance 4