Oh, there are so, so many. The bathtub explosives, the “Chicago Way” episodes, the story about the pizza, the story about the guy forgetting his pants, him mocking that racist chick…
I guess if I had to pick just one though, I suppose I would go for the one where he talked about how to run a Babylon 5 campaign that would keep people from using foreknowledge of the show to ruin the fun. I think it was the episode titled All Jedi Or No Jedi.
I brought that whole hypertime/people can play the role of X but they can’t Be character X thing, and how he said don’t assume, that the Morden in his game “could be an encyclopedia salesman for all you know” up to my step-father, and that led to him doing just that, starting a B5 campaign using those rules. What’s really cool about that game is I’m in it, and (with permission) the character I’m playing is based on Tara from What The Fuck Is Wrong With You. :)
I think that particular episode was "Hey Fatty! Let’s Go To Mordor!" since I remember him talking about Babylon 5 in that episode but I could be wrong. I know he talks about similar scenarios in many of the videos though.