A Funny Thing Happened on the Way to the Forum (1966) is, quite probably, the funniest film ever made. Is there anyone who hasn't already seen it? I love this one! It stars Zero Mostel, Phil Silvers, Jack Gilford, Buster Keaton, Michael Hordern, Roy Kinnear, Jon Pertwee and Ingrid Pitt. The music is by Stephen Sondheim.
TCM has an overview. Rotten Tomatoes gives it a score of 82%.