Going by the decades:

1880's: Peter Jackson
1890's: Bob Fitzsimmons
1900's: Sammy Langford
1910's: Jess Willard
1920's: Harry Wills
1930's: 'Tiger' Jack Fox
1940's: Jimmy Bivens
1950's: Sonny Liston
1960's: Oscar Bonavena
1970's: Ossie 'Jaws' Ocasio
1980's: Tony Tucker
1990's: Tommy Morrison; (you can click me later Brad )
2000's: Wlad Klitschko