John Collier -- which one do I pick? The super-short "The Chaser" (shorter than "The Clock"!), which is a disturbing story about why love potions only cost a dollar? "Evening Primrose" has been so over-adapted, -anthologized, and -remembered it was pointless. And "Bottle Party" while not really a horror story, has one of the most horrifying and disturbing last lines ever written.
But I like "Thus I Refute Beelzy", if only because it uses a sitcom format to tell its story. One that ends with a nasty, nasty...bite.