The Diablo games. Over the past five years, I have played those games for a total of at least several hundred hours, probably at least a thousand hours. I might actually have played Diablo II: Lord of Destruction for a thousand hours by itself. I still play it, I'm addicted, I can't wait for Diablo 3. And don't tell me they're not going to make it, they have to make it. They just have to.
EDIT: I just saw that everyone else was posting about one sitting and realised that's what I'm supposed to do, so here it is. I once played Halo: Combat Evolved for 16 hours. Didn't stop to sleep, eat, drink (I had a beverage and took a sip during some of the loadscreens, but that's it) or anything. My eyes were locked to the screen and my ass glued to my foam chair (you would not believe how comfortable that thing is, best 3 bucks my parents ever spent on me) the whole time. I only stopped because I had to clean the house before my parents got home or I'd get in a lot of trouble.