Again, another toss up, particularly if you're talking about things with great rewatchability.
Jesus, how do you pick from such great and wonderful films made over history? However, I have to confess my personal tastes tend toward hybrid movies. Still, do I choose from
Ace in the Hole
The Assassination of Jesse James by the Coward Robert Ford
The Friends of Eddie Coyle
Thomas Edward Seymour's moving indie Everything Movies Alone
John Sayles' mystery/drama Lone Star
Sam Peckinpah's Straw Dogs
Peter Medak directing Marat/Sade: (The Persecution and Assassination of Jean-Paul Marat As Performed by the Inmates of the Asylum of Charenton Under the Direction of The Marquis de Sade)
But I think it really comes down to John Huston's adaptation of Flannery O'Conner's Wise Blood
I seem to be very bad at this "One Favorite" thing.