It is not going to be possible for me to name the exact longest session that I ever played or ran because there were so many weekend-long games during high school. It would have to be either AD&D 2nd edition or WEG Star Wars. It is more likely to have been AD&D because I was running Star Wars every week and some times more than once in a week, so we did not tend to do marathon sessions with that game.
I can tell you about a session that ran so long that the GM fell asleep. We were playing Rules Cyclopedia D&D and one of my players was running the game, he almost never DM'd and it was going really well. It was not a dungeon crawl, it was a mystery. We were closing in on the culprit and suddenly, in mid-description, the DM's head goes down and he starts to snore. We nudged him to try to wake him up long enough to finish the game and the only response we got was, "She's got your mother's dress, but she doesn't have the body."
When he woke up the next day, he couldn't remember anything. He only remembered the beginning of the game, and was apparently making it up on the fly. We never found out who the murderer was.