Probably FFX, from like 6pm (after dinner) till 3am...the story was that engrossing, so that was 9 hours. That was Summer 2003 when I was 15.
DMC runs a close 2nd. I beat that game all in one go back in October 2001 before I owned a PS2. I rented a PS2 along with DMC when it came out and I didn't have a memory card. So I beat it all in one day, only taking one 30 minute break for Dinner. I started at about 1pm after lunch and finished around Midnight. I would have been 14 at the time, as it was October 2001.
There have been various other games where I have done 5-6 hour sessions, such as Kingdom Hearts, Xenogears, other FF's and other RPG's. I also used to have long sessions with Brigandine.







