For me it would be GTA IV, I've been addicted to the GTA series since I first got Vice City and then San Andreas (the greatest game ever), I think I put more than 300+ hours on GTA IV just by driving around and looking at the details of buildings and just walking around pretending I was a real person. It came to the point where I spent an entire day driving one of the buses around the business area, with a constant route to look how that would work, I can't believe I'm admitting I did this, I'll show myself out now.
Which one was THAT game for you?
Which one was THAT game for you?