I'm looking for an RPG that captures the feeling of 'Monty Python and the Quest for the Holy Grail' (preferably fantasy, although if there's a system like this that is settingless or for another setting and easily adapted, that'd be good too). It must be suitable for a one-shot, so anything with a core system I can't explain to my players in under 5 minutes is out.
The things I'm looking for are collaborative world creation/editing and rewards for humour (so it's better to have a Holy Hand Grenade of Antioch than just a Flask of Explosion, even if they're mechanically the same, and the player who came up with it gets a small reward of some sort). Ideally, there'd also be a way for players to control NPCs a little bit (maybe tied in to the collaborative world-building), but that's the sort of thing I can add on myself if necessary.