Sound Off! What games have you spent the most time playing?

Recently, I took a look through my Steam library and realized that I've spent a lot of time playing games. From old standbys like Civilization to more recent titles like Kerbal Space Program, there's hundreds hours of my life sunk into some of these titles. Let's not even get started on colleagues and friends who've played thousands of hours of WoW and Dark Souls. Which games have you found yourself obsessed with and how many hours of your life have you devoted to them? Head over to the Engadget forums to share your top games or post about them below.