The Godfather Part 2 (1974)
This gangster film was directed by Francis Ford Coppola and both a sequel and prequel to the original Godfather film.
It also stars Al Pacino, Diane Keaton, Robert De Niro, and Talia Shire).
Duvall stars as Tom Hagen. Duvall also starred as Tom Hagen in the original Godfather but I listed this one instead of that one because I consider this one to be the better film of the 2.
This film won the Academy Award for Best Picture.
Apocalypse Now (1979)
This film, also directed by Coppola, is (as if you've never seen this film, c'mon, seriously?!?) a Vietnam-era war film.
Duvall stars as Lt. Col. William Kilgore, the commander of a calvary unit. He says the famous line "I love the smell of napalm in the morning."
This film also stars Martin Sheen, Marlon Brando, Laurence Fishburne, Dennis Hopper, Harrison Ford, Scott Glenn, and R Lee Ermy, among many others.
This film was nominated for the Academy Award for Best Picture in 1979 but somehow lost to Kramer vs Kramer (really?!? something is just not right there).
The Apostle (1997)
Not only did he star in this film (as the main character Apostle Sonny Dewey) but he directed it as well.
Also starring in this film were Billy Bob Thornton and Farrah Fawcett.
For his role, Duvall was nominated for an Academy Award for Best Actor but lost to Jack Nicholson for his role in As Good As It Gets. I would have chosen Duvall over Nicholson based on their roles personally.