The Community clip episode is great, but I think the best one has to be Avatar. The original one, Ember Island Players, not the lame-ass actual clip episode in Korra. An in-world summary of the events thus far as portrayed by actors who don’t have complete knowledge. With in-jokes referencing various aspects of the show’s production, like how Toph was originally meant to be a burly man, and how Aang, as a young boy, would have been voiced by a woman in most cartoons.