Least successful not already mentioned (and maybe all-time):
Luise Rainer - the posterchild for this before Cuba Gooding. A few forgettable films after her 2 awards then a 50 year break to make one more film. I guess she could make a comeback at age 102.
Geraldine Page - Made 3 forgettable films then died after The Trip to Bountiful.
Jessica Tandy - Except for Fried Green Tomatoes, a string of minor films after her Oscar.
Reese Witherspoon - What has she really done since 2005 besides Va’Ginormica in Monsters vs. Aliens?
Now for the men
Emil Jannings - Raises a question to the OP of definition of success since he starred in a ton of films in Germany once talkies came out but nothing in the U.S. after that.
Ronald Colman - His best work was before his Oscar. Only 3 films after that
Art Carney - Was in the Star Wars Holiday Special