GTA4 has the unlucky position of being released many months ago and so everyone here has pretty much forgotten that it's an incredibly solid game, with a fun campaign and pretty much endless side stuff worth doing. Calling a game the "best" is incredibly subjective. It's the game that made the most money, so from a business perspective sure why not?
Best game to me was definetly MGS4, but I'd give Fallout 3 props, and I give GTA4 respect as well, not to mention Pixeljunk Eden, LittlebigPlanet, Valkyria Chronicles, Resistance 2... a lot of really top notch games. To say which is best is kinda like asking if red, green or blue is best. You can make the case for any of them and it just really matters what your personal preference is.
I'm actually glad there's nothing really on the horizon til Killzone 2 comes out, because I'm pretty content with the games I already have.
By the way, I'm surprised nobody mentions Braid anymore. When it came out, everyone's talking about how it's their GOTY, then a week later everyone forgot it. Haven't played it myself, but I will. Just thinking out loud, maybe people have such low expectations for downloadable titles they didn't know what to make of it.
By the way, with regards to LBP, to me it's kind of a 2nd wind game. I tried it for like an hour, shrugged it off for a while, then came back to it and just loved it. It's not for everyone and you kinda have to be in the mood for it... but if you like making your own content, trying other people's content, collecting items, playing with friends and strangers, ext.. it's very easy to just start playing it and having it just take you over.
Oh yeah... and I know Disgaea 3 isn't GOTY quality or anything but production values aside, I had as much fun playing it as any of the big dogs, even if it was a beefed up PS2 game with a somewhat repeatability formula. Just really wanting to play an old school styled TRPG and that coming along made it a great experience.
On the other hand, maybe some of us didn't really feel the need to play another sandbox GTA style game.