The Elder Scrolls IV: Oblivion
Spent countless hours on that game, and Morrowind, to put them together I would have to say I have around 2000 hours into those games, I got so good at oblivion to the point that I really wouldn't even want to play it anymore because it's not a challenge, but then a few months later, I wanted to play it again, and I couldn't find it and still searching for it to waste more time in my life on XD.