In 92 years of Oscar, 82 men had won the Best Actor Oscar; some more than once.
Not all of the honored performances were of high-caliber. Below please find my list of the weakest (least deserving) performances by the Best Actor winners.
I have arranged them in chronological order
Warner Baxter, In Old Arizona, 1929
Spencer Tracy, Captains Courageous, 1937
Paul Lukas, Watch on the Rhine, 1943
Jose Ferrer, Cyrano de Bergerac, 1950
David Niven, Separate Tables, 1958
Lee Marvin, Cat Ballou, 1965
Cliff Robertson, Charly, 1968
Richard Dreyfuss, The Goodbye Girl, 1977
Roberto Benigni, Life Is Beautiful, 1998
Eddie Redmayne, The Theory of Everything, 2014