The Conversation would probably make my top five films of all time, he's just fantastic in it. One film people often overlook his performance in is Bonnie And Clyde, I'm not a massive fan of the film, but he's the best thing about it. Then there's Scarecrow, alongside Pacino, Night Moves, Roeg's Eureka is another unheralded performance of his.
For my money, one of, if not THE best American actor of all time.