I have to agree with Postjack, Oblivion was the game that I kept playing over and over, due to the variety of characters, quests, add ons and ways that you can play.
And Fallout 3 was okay, but definitely doesn't have the replayability of Oblivion. That is why I'm looking forward to the release of Skyrim.
Other than that, I find myself keep being drawn to the Call of Duty multiplayer games.