I can't say Fallout 3 even though I would like to cause I've never played it.
I have to go with Fable II. I really had a lot of fun with that game, and one of the few games this year I played from start to finish without stopping to play another game in between. I did all the side quests and really had a lot of fun making all the decisions that they throw at you. Also, the most fun I've had (that wasn't Saint's Row 2) was killing innocent villagers with ma crazy magicks! The combat system is a good mix of simple but complex if you want it to be. Sure the main story isn't very long, but when you're doing everything, exploring the world, and having fun doing it, it turns out to have its legs.
As for GTAIV, it's very interesting. This was my original GotY pick but it's also my biggest disappointment pick. The story is great and the realistic word is amazing, even for someone that lives in NYC. The characters were great and funny, and it also had a couple of decisions that had me thinking if I made the right decision. However, I just got tired of the story. After you finish the Dwayne and Playboy X missions and get to the Mafia type missions, it just gets really downhill. I couldn't even be bothered to finish the game until about 6 months after I reached that point. The game has its quirks and awesome facets, but I just did not have much fun at all after the initial thrill of having such an awesome realistic world. It still has the best writing though, no doubt.
I played Gears 2 and while I did love it initially, even beating the game about 5 times, twice on hardcore, once on normal, and twice on insane, there's something about it that just doesn't even want me to call it a GotY candidate. It's a great game with it's own great moments, but the broken multiplayer really pissed the hell out of me. Maybe cause it wasn't such a big leap from Gears 1 that it doesn't have such an impact on me, but they did add a lot of things that made it more enjoyable than the first Gears.
Also, I wanna mention Dead Space, cause that game is just fun. It's not very scary after awhile, but the sound effects and direction of the game with attention to detail is a big plus for me. It's a cliche type story, but I don't really know how many kinds of stories you can have about aliens on a spaceship. They still did it great. As they said on a 1up (RIP?) podcast, it's kind of like a greatest hits of survival horror games. BTW, if you're an avid gamer, I recommend to start on Hard difficulty, as Normal difficulty is pretty easy.
Overall, this was a great year for games. I've already heard some people saying next year won't be as good, but that's what they said about 2008 after the deluge that was 2007. I bet this year will be another great one with people once again making schedules just to play all the releases. Last year, we saw a lot of the games get stuck in the holiday season against some crazy competition and because of that, they were just overlooked. Examples are FarCry 2, Prince of Persia (sort of), and Mirror's Edge. They should really think about spreading these games around the year. GTAIV has shown that even with a release in late April, that you can still sell millions of copies, so why stick your game in the competition with the biggest guns in the business when your game is great but can't compare?