Worldwide? Very likely to be Wii Fit if Nintendo can meet the demand.

GTA4 defientely will sell a LOT.
FFXIII, if it comes in november and WW.
Also GT5 has good chances, but also if comes in november and WW.

Grand Theft Auto: San Andreas 15.22m
Grand Theft Auto: Vice City 14.20m
Grand Theft Auto 3 11.60m

Gran Turismo 3 14.87m (first GT on ps2)
Gran Turismo 4 8.87m

Final Fantasy X 7.95m (first final fantasy on ps2)
Final Fantasy X-2 5.21m
Final Fantasy XII 5.01m

Super Smash Bros (N64) - 5.55 mln. since 1999 with around 20 mln. N64 on launch.
Super Smash Bros Melee (GC) - 6,58 mln. since 2001 with around 3 mln. GC on launch.

Mario Kart 64 9.87m
Mario Kart DS 9.08m
Mario Kart: Double Dash GC 6.60m

Metal Gear Solid 2 5.56m (first mgs on ps2)
Metal Gear Solid 3 3.96m

Guitar Hero II ps2 3.41m +x360 1.88m = 5.29 mln.
Guitar Hero III ps3 0.61m + wii 1.29m + x360 1.78m + ps2 2.55m = 6.33 mln.

Resident Evil 4 ps2 2.53m + gc 1.66m + wii 1.34m = 5.53 mln.

Pro Evolution Soccer 6 (WE10) ps2 3.93m + x360 1.08m = 5.01 mln.
Pro Evolution Soccer 2008 ps3 1.11m + ps2 2.72m + x360 0.75m = 4.53 mln.

Gears Of War 4.87m

Call of Duty 4 x360 3.21m + ps3 1.61m = 4.82m
Call of Duty 3 x360 1.95m + ps3 0.81m+ ps3 0.55m + wii  0.35m = 3.66m

Dragon Quest VIII Enix 4.69m
Dragon Quest VII Enix 4.33m

Tekken Tag Tournament 3.94m (first takken on ps2)
Tekken 4 3.31m

The Getaway 3.37m

God Of War 3.15m (first gow on ps2)

Devil May Cry 2.77m (first devil may cry on ps2)

Resistance: Fall of Man 2.25m

LBP sales, Killzone 2 sales - are unpredictable.

^so what do you think will be the best seller

The order in what i placed games is pretty much showing how will sell their sequels. =)
And wii fit sales are unpredictable too, though it`s not even a game, but as a software soft should be around 5-7 mln.

@ Crazy Man,

You probably shouldn't judge how well a game will sell based on previous versions of the games, mainly SSB and GTA.

I would bet it'll be GTA4, SSBB, GT5

My predictions are for WW:

1.) WiiFit
2.) SSBB
3.) GTAIV(both console versions)
4.) Mario Kart Wii
5.) Super Mario Galaxy

Hmm.. after some reconsideration, my estimate for GTA4 is probably a bit low. It'll definitely sell way less than the previous parts, but even half of their numbers may be enough to make it the best selling non-bundled home console game.

GTA4 definitely has a chance at being number one, although my money is still on Brawl. I still believe no exclusive on the HD consoles has a chance to appear in the top 3, and the chance of one making it to the top 5 is under 50%.

1. GTA IV = Because of it's huge attraction and the game being multi-platform.

2. MGS4 = PS3's main exclusive AAA game.

I'm thinking its going to be StarCraft II. 6 million sales in South Korea alone.