Some of my favorites

-Jack Cassidy in Murder in the Doll's House

-Jackie Coogan and Arlene Golonka in Sing a Song of Murder (This was is well written because they didn't conspire to murder, but took advantage of a situation. Arlene's character is extremely vile and callous.)

Terri Nunn in Academy of Evil gets my vote. Her acting ability was off the chart. She auditioned and almost got the role of Princess Leia in Star Wars. When you watch the episode you can really see why. Its just Barnaby Jones but she really soared at this evil, manipulative, yet sweet and charming wannabe killer.