Admittedly its subjective, but how many best actor/actresses winners have not enjoyed much success (both before and after the award).
Adrian Brody; he is acting, but nothing which has found much success or acclaim.
Halle Berry: hard to say what she has done since Catwoman.
Jamie Foxx hasn’t done much since Ray, although he’s starring in Tarantino’s next flick, so there’s still hope.
Roberto Begnini has been MIA since Life is Beautiful, and good riddance.
Going back a bit further, F. Murray Abraham never managed to follow up on Amadeus. He’s working, but it’s largely bit parts and C-movies.
Cuba Gooding Jr. - won an Oscar for Jerry Maguire in 1996. His career after that is all over the map: a few notable roles, a few Razzies, and a couple of straight-to-DVD efforts.
Without doing any research to support my guess, I would think the reverse list would be more impressive: “Which Actors/Actresses Have Done Their Best Work After Winning An Oscar?” I’m struggling to think of any!
Successes
Katherine Hepburn was very successful after her first two wins.
Dustin Hoffman
Philip Seymour Hoffman
Agreed. Also,
Meryl Streep!
Marlon Brando (along with some utter tripe)
Gene Hackman
He has had some good work in Italian cinema though. He is also in Woody Allens next flick.
Jack Nicholson.
Marlee Matlin hasn’t done much other than West Wing after winning in 1986, has she?
Ditto Brenda Fricker since her My Left Foot win in 1989.
Or Marisa Tomei since My Cousin Vinny in 1992.
Or Mira Sorvino since Mighty Aphrodite in 1995.
Too soon to say about Mo’Nique, who won three years ago for Precious?
For the record, the OP asked for Best Actors/Actresses, not Best *Supporting *Actors/Actresses.
Depends on how you define “success” I guess, but Marisa Tomei has done some excellent work (The Wrestler, Slums of Beverly Hills, Before the Devil Knows You’re Dead).
Louise Fletcher won a Best Actress Oscar in 1976 for One Flew Over the Cuckoo’s Nest. She hardly made any movies for several years after that and there were films likeThe Lady in Red, *The Cheap Detective *, and Exorcist II: The Heretic. None exactly Oscar bait.
Eventually wound up doing generic work on TV shows.
In the late 70s people were wondering whatever happened and she said that after she won, she stopped hustling for work since she expected the phone to start ringing. It didn’t.
Jennifer Hudson. Best Supporting Actress for Dreamgirls. Now plugging Weight Watchers.
Helen Hunt should probably be on this list.
It is often the case that the Best *Supporting *Actress/Actor is never heard from again or just has bit parts after winning, but the OP asked about when this happens with the *leading *actors.
I think the “good” list ought to be, "Actors/Actresses who rose to fame by winning the Best Actor/Actress Oscar and then continued to have success"
This eliminates winners who were already famous or long overdue.
Reading this thread reminded me of a Fark photoshop contest from a few years ago.
The contest was basically “photoshop whatever happened to Anthony Michael Hall” and the winner was a guy who just put up a poster of The Dead Zone with Anthony circled and “I’M RIGHT HERE!” in big red letters.
That’s who I was going to say. You’d be hard pressed to find someone who has done less after winning Best Actress.
Peter Finch never made any more films after winning for Network. 
It’s hard to say that Al Pacino is not successful, but since winning his Oscar for Scent Of A Woman, has he made anything that wasn’t crap?
Jon Voight went from Coming Home to Anaconda.
Nicolas Cage, well…
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