The least deserving men who have won the Best Actor Oscar over the past 89 years, often for a mediocre performance, and due to other, extra-curricular factors.
Many of these men have been nominated for, but did not win, great performances. Jack Lemmon is a prime example.
Worst Performances, Chronologically
Lionel Barrymore, A Free Soul
Spencer Tracy, Captains Courageous (he won a second, consecutive Oscar for a better performance in Boys Town
Paul Lukas, Watch on the Rhine
Jose Ferrer, Cyrano de Bergerac
Ernest Borgnine, Marty
David Niven, Separate Tables
Lee Marvin, Cat Ballou
Cliff Robertson, Charly
Jack Lemmon, Save the Tiger
Art Carney, Harry and Tonto
Richard Dreyfuss, The Goodbye Girl
Eddie Redmayne, The Theory of Everything
If I have to choose three, they would be David Niven in Separate Tables, Lee Marvin in Cat Ballou, and Richard Dreyfuss in The Goodbye Girl