Demon's Souls at around 250 hours. I wish Animal Crossing on the DS kept track of time played because it could be a serious contender to beat out Demon's Souls. Anyways Dark Souls comes in second that I know of, at 212 hours.
I envy people that can enjoy a game for 500 hours or more. Talk about getting your money's worth.
I don't tend to spend a lot of time on one game. I'm going to guess that Skyrim would be the most at roughly 150 hours. Second place would probably be Halo 2, which I used to play every day back in middle school. It was my first online game ever.