Was he the greatest male vocalist of the 20th century? I don't know but he has to be among the finalists.
According to a recent article, Frank Sinatra confronted Mario Puzo, the author of "Godfather". According to the story, Sinatra berated Puzo one night in a Los Angeles. Sinatra was offended that he may have been the inspiration for Vito Corleone's godson or Johnny Fontane.
We will never know but it makes for a great story.