You can't ask a film lover this, because there are the directors one knows are "the greats", and then there are ones who may inspire you and personally affect you, but everyone else may hate them. So I kept my list all-American baby! 

Orson Welles
Alfred Hitchcock
Buster Keaton
Howard Hawks
John Ford
Charlie Chaplin
Stanley Kubrick
Billy Wilder
Fritz Lang (All his American films are masterpieces)
Woody Allen