Most recently it's been Oblivion(well over 200 hours) and MLB 08's Road to the Show feature.
All time, I'm with Calamity, I spent more time with Tetris than any game since. I seriously doubt any game will ever come close.
Honorable mention has to go to Solitare and Minesweeper at work.
I've already been warned by a friend, though, that Disgaea 3 will be a legendary timesuck. So I'm looking forward to that, albeit with some trepidation.