I think either Wii Sports, Wii Play, Wii Fit, or New Brain Training.

SSBB will be on top then gta4 then mariokart

 I'd agree with all of these, and add SSBB to the list. Basically, all four of these could end up leading. 

Why do you say that Gears 2 would sell better now? When it came out it was THE Game, now it has so much compition. In 06 the best Next Gen game out was Zelda and that was more like 1.5 Gen since it was just a cube port and polish.

TOO HUMAN

Number one ,it will sell well next year and will crush God of War sells !

Probably GTA4 since 360 and ps3 get it, i would say GT5 but its only ps3 which limits the userbase.

I'd say in the long run, like a year or 2 though, GT5 will sell at least 5 mil.

I'll narrow it down a bit for sanity's sake... No expansions, no bundles, and no handheld games.

With that said, I only see these games having a shot at the top:

1. GTA IV
2. Mario Kart
3. SSBB
4. GT5 (unlikely that it will launch WW in 2008 IMO)

Killzone, MGS4, and some of the other games I see listed here don't have a snowball's chance in Hell of topping those games. The only FPS that could even think of hanging with those four games is Halo and even that hanging with GTA and GT is unlikely. It's hard to accurately judge the Wii's install base so MK and SSBB just turn mediocre sales (4-5m) just as easily as they could go ballistic (8m+).

1-GTA4
2-Wii Fit
3-Wii Play
4-SSBB
5-Nintendogs
6-Brain Age 2
7-GT5
8-Madden 09
9-Mario & Sonic
10-Mario Galaxy

They both have Kingdom Hearts games.  (Although IIRC the PSP one comes out in '09)

OT: Smash bros or GTA

If we are talking about this year alone, WW sells, consoles only....

GTA4 (combined two platforms)
Mario Kart
WiiFit
Next Guitar Hero or Rock Band game

Couple of caveats. SSBB, I honestly believe is going to be another Metroid Prime, where only the core will care about the game and will never get past 2 mil. Same goes for MGS4. GTA4 is released on two platforms and will end up selling over 5 million WW. WiiFit will be supply constraint just like the Wii. Mario Kart will sell cause its a Mario brand game and a family friendly game that will appeal to Alpha moms better than the wild out of control action SSBB. Rhythm games has the hot hand for the last year and half, and a lot companies will try to capitalize on that. If sells pick up for Rock Band when the sold alone peripherals come out , GH will answer back this year with something big.... Most of the shooters will cancel each other out, not know which will be the big one this year. Don't forget about the X factor Spore that is coming out. If it picks up like the Sims, then it may shatter them all.