A real favourite of mine. If Sirens was fan-service to get the ball rolling and Phantasmagoria was BF showing that they could do a TV serial on audio, then Whispers was their first truly ground-breaking story.
Not only is it a tale which only works on audio, as others have mentioned, but it manages to avoid the kind of in-jokes and references which plague a lot of fan fiction. No historical side-gags or wandering classic monsters, just a clever, tight, story told in a new and exciting way.