Kind Hearts and Coronets (1949) - not really obscure since it was the crowning glory of England's Ealing studios, but perhaps not so well known in America. A ludicrously dark and funny film, in which Alec Guinness - who was a talented and accomplished actor, not just Obi Wan Kenobi! - plays eight different characters.
"Louis Mazzini's mother belongs to the aristocratic family D'Ascoyne, but she ran away with an opera singer. Therefore, she and Louis were rejected by the D'Ascoynes. Once adult, Louis decides to avenges his mother and him, by becoming the next Duke of the family. Murdering every potential successor is clearly the safest way to achieve his goal... "
http://www.imdb.com/title/tt0041546/
(#147 in the top 250).