So many to choose from but probably when I was a teen we'd go over to a mate's house in Kirriemuir and I would run AD&D Dragonlance, for about 16-20 hours straight. Now I'm a DM who likes to have written out a loose script, so by about the 12 hour mark I'd have run out of plot.
Gregor: Alright we walk into the alley.
Me: Yup. You walk straight into the alley.
Hoppy: Do I hear anything.
Me: Not at first. But then suddenly behind you there is wicked laughter.
In the other room Gregor's Wicked Stepmother then proceeds to laugh in a manner that would make the Witches from the Wizard of Oz envious.
And sadly that was an accurate indicator of her personality.