Killzone2, Resistance 2 and MGS4 are usually brought up at this point but none of them have the mass market appeal to drive sales, just because they're the most hyped by the hardcore gamer audience or technology entuhsiasts.
The Game for this year is obviously GTA 4, and it's not exclusive to any platform. And even though Sony probably has a bunch of 1st party titles coming up, but their two biggest titles, Final Fantasy and Gran Turismo, will not be released this year.
I'd expect that the game lineups are pretty much comparable in the end... There's certainly not going to be an edge on either side this year that's comparable to Halo3. And all the rest of the best selling games of 2007 has been multiplatform (COD4, AC, GH3); but I'd rather say that 2008 pales a bit in comparision altogether, at least so far.
Edit: to clarify my opinion a bit further, I'd consider a game to have significance if it sells about 4-5 million copies at least. This means that I have to add Mario Galaxy to the above four; and also that MS has only the Gears sequel in the works that can have such a great effect.