When I think of Existentialist film, I think of the two polar examples of Bergman and Antonioni. Bergman's focus is primarily on what Heidegger described as a main focus of Existentialism as Being with Others. Antonioni's focus was more on alienation, specifically in relation to society and even the landscape. In between I should also mention Truffaut as an Existentialist. Shoot the Piano Player for example is a film heavy with Existential content.
Of course one could argue that really most if not all Continental post WWII directors were influenced by Existentialism. Fellini, Malle, Godard, Bunuel, Rosselini. But Bergman, Antonioni and Truffaut first come to my mind.