"What is the narrowest genre RPG you have ever played? How was it?"
Yeah, definitely Pendragon. A genre of RPG that exists entirely to run games set in Arthurian legend, where if you're not a knight (or at least landed), your life is hella boring, meaning you're an NPC.
And god is it fun.
There's always that adage about how restrictions having the ability to enhance creativity, and man does that ever work with Pendragon. Everything from rolling randomly to see how your family have interacted with the last hundred years of backstory, to creating your own coat of arms, to roleplaying within the strictures of the trait/passion system, if you have a player who doesn't exactly know 'how' to roleplay, I'd seriously suggest initiating them with Pendragon, or at least taking some of its mechanics with you to the rest of your games. And, because it's d100/d20 based, it works well with both BRP and D&D (also Pathfinder, D&D's bastard genius prodigy).