I don’t really have a roleplaying one, unless you’re going to count “don’t be a dick” which should be a staple of all roleplaying systems.
My favourite house rules pretty much all come from my days playing board games at Watt Gamers, where you got rules like “always gang up on Andy” where they weren’t “rules”, but were the teasing results of games where if you didn’t gang up on Andy, he had a tendency to create massive fleets and take everyone else out.
Footnotes [ + ]
|1.||↑||Be a bastard, don’t be a dick. Be evil, be selfish, take advantage of situations, and do it all IC – don’t be a dick about it OOC.|