Miskatonic University Campus

I'm going to throw my hat into the ring and say "The Rats in the Walls" -- it's got that eerie gothic sensibility that evokes something of Poe, but is wholly Lovecraftian in the way it plays with the narrator's rapid descent under the weight of his lineage's cognitive and physiological degeneracy. It's also almost a blueprint for a workable CoC scenario. Runner up in the short, short fiction category would be "The Statement of Randolph Carter" which shoehorns so many of Lovecraft's themes into just a few spare pages and leaves the reader with that wonderfully off-putting conclusion. I think you have to treat the novellas as their own category.