Wii Sports and maybe Wii Play if you count them, but they're sorta cheating by being bundled. I'm also ignoring stuff on the DS, just because I haven't been following them much, and a Yearly section for Europe isn't readily available to better analyze Brain Training etc.
SSBB has a very good chance of coming on top of the non-bundled "real" game pile. It has huge appeal to both casual and (especially) hardcore audiences, not to mention how insanely well it did on "just" the Gamecube.
Wii Fit is a bit of a mystery, but has the potential for selling truly ridiculous amounts. I'm probably underestimating it a bit, since I really don't give a damn for it.
GTA4 is the game I think most people are seriously over-estimating. The last few parts did very well because they were games everyone could play on the console everyone had. I've typed a long explanation for this a million times, which I have no intention of repeating here, but basically most people have trouble playing a game where you shoot aliens, lead armies etc, but almost everyone can relate to a regular guy on the street, getting caught in a life of crime. I don't think the hordes of people who bought the really succesful ones on the PS2 generally have HD consoles, and GTA4 will sell 5 million copies during its lifetime at best. That's still very well, but definitely not the best selling game of the year. Of course I never liked the GTA games, so I don't exactly understand the appeal in them. We'll see if I was right, or very wrong soon enough.
GT5 has much of the same issue, since obviously racing games are very casual friendly (compared to shooters, strategy games etc). Still a million seller, but without a doubt the least selling part of the main series.
Now that I think of it, there probably won't be a single PS3 or 360 exclusive in the top 5 best selling games this year.
Starcraft 2 might have a chance, but I'm unfamiliar with PC sales.