So much good competition this year but ultimately I went with Fallout 3 GOTY since it's one of my favorite games and you can't beat all that content in one package.
Borderlands and Dragon Age are so great for their replayability and just RPG nature/having to level up which makes the game even longer. It took me 100 hrs to beat DAO once with a level 20 Warrior and I have 2 more classes to go. And recently I got Borderlands but it's going to take a long time to hit level 50 with my Soldier and that doesn't include the other 3 classes.
God of War Collection, Metroid Prime Trilogy, and Sonic's UGC are also notable for packaging multiple individual games together that up your play time considerably.
MW2, RFG, and Uncharted 2 also deserve recognition for having lengthy single player/co-op components as well as even lengthier multiplayer components which also makes them endlessly replayable.
This may be the most difficult category yet! AHHHHHHH! I'm so torn but will stick with Fallout 3 since it was robbed last year IMO which in turn might rob a worthy nominee this year. Sigh.