The best would be "The Man With No Name" trilogy by Sergio Leone & Clint Eastwood. A Fistful of Dollars (1964), For a Few Dollars More (1965) and The Good, the Bad and the Ugly (1966).
For more modern films I would definitely go with There Will Be Blood, Unforgiven, The Assassination of Jesse James by the Coward Robert Ford, Django Unchained, True Grit & 3:10 to Yuma.