I still say GTAIV.

GTAIV (by a country mile hopefully).

 Haha, that guy is the man. Convincing a customer not to sell their game.

Its a close call but I'm gonna say GTA4 because it has two platforms to sell on.

GTA4 will win

since you combined 360 and PS3 as one for GTA4, then no doubt it will be GTA4. I think SSBB will be good but i don't think it will have the sales that everyone thinks. Like I think GTA4 for just 360 will outsell it, not the PS3 version alone though, unless it has a larger install base by then.

I honestly have to go with GTA IV because I spent over 400 hours on San Andreas. I have around half that for SSBM.

SSBB will have a good launch date but GTA4's launch will be better (it could even reach SSBB lifetime sales). but Brawl will beat GTA4 with strong legs

I believe that SSBB will be the year's best seller.
Look at the numbers:
Super Smash Bros. Melee sold roughly 1/3 the amount on a userbase that was roughly 1/6 that of the PS2+360.
Attach Rates:
GTA: San Andreas sold roughly 17 million (combined PS2 and Xbox), for about a 12% attach rate
SSBM sold roughly 6.5 million, for about a 30% attach rate
So even if GTA somehow gets 15%, a slight increase in attach rate, and SSBB drops down to 20%, a massive decrease, Brawl still wins.
Just using today's numbers:
19.45 million Wiis*0.2=3.89 million
Combined 360+PS3=24.73 million
24.73 million*0.15=3.7 million
The Wii will also slowly catch up to combined 360 and PS3 sales, and eventually pass them. The gap between the titles will slowly grow larger, though undoubtedly they will both still sell really well.

In the long run Brawl will beat PS3 and 360 sales of GTA4 combined.