If you ever want to see a real (little-known) gem of his, check out "The Rose Tattoo". Its from the same period as "Trapeze" and its absolutely hilarious. I first saw it in a college class and fell in love with the movie.
Several other overlooked but excellent Lancaster roles / movies :
Come Back Little Sheba
Elmer Gantry (A star studded filled supporting cast)
The Swimmer
The Rainmaker (original)
Don Cardi
Five - ten years from now, they're gonna wish there was American Cosa Nostra. Five - ten years from now, they're gonna miss John Gotti.