'The Godfather'

Considering the schlock he does now, it's really easy to dismiss Al Pacino. But he is easily one of the top 3 actors of the past 40 years. His metamorphosis in "The Godfather" is arguably the best part of the entire film.

Furthermore, one of the top 5 actors of the past century, Marlon Brando, is a certifiable nut. In his dying days, I picture him being a mix of Vito Corleone and Dr. Moreau.