Considering my viewing habits there's a lot I have only watched one or a couple films from and I should really get into, but from the ones I haven't touched yet:
Bernardo Bertolucci
Elia Kazan
Erich von Stroheim
Jacques Tati
Jacques Rivette
John Frankenheimer
Louis Malle
Mel Brooks
Ousmane Sembene
Wim Wenders
You're in for a treat! -- I'll list some movies, and hopefully you like them. If you want to see my favorite documentary, check out Louis Malle' "Place de la Republique" - he goes around Paris, asking people provocative questions...
Last Tango In Paris
On The Waterfront
A Streetcar Named Desire
Birdman of Alcatraz
Alice in the Cities
In the sense of directors I want to get into but can't seem to, I would say Wim Wenders is mine. He's hit and miss with me, so I haven't really looked for his movies in along time.