In each of the five on the list below, the main character is nutty. Craziness is what drives the story. To narrow the field, I’ve limited it to movies that are essentially about characters who are steadily getting more detached from the supposedly sane reality around them. Since it's hard to find the sane world in the midst of a war, movies set in war are not included on this list of five, which is presented in alphabetical order.
"Aguirre, the Wrath of God" (1972): Directed by Werner Herzog; Cast: Klaus Kinski, Helena Rojo, Del Negro
"Network" (1976): Directed by Sidney Lumet; Cast: Peter Finch, Faye Dunaway, William Holden
"Repulsion" (1965): Directed By Roman Polanski; Cast: Catherine Deneuve, Ian Hendry, John Fraser
"Taxi Driver" (1976): Directed by Martin Scorsese; Cast: Robert DeNiro, Cybill Shepherd, Peter Boyle
"Wise Blood" (1979): Directed by John Huston; Cast: Brad Dourif, Harry Dean Stanton, John Huston