Abbott and Costello perform the classic "Who's on first?" baseball sketch in their 1945 film "The Naughty Nineties" first performed as part of their stage act.
William (Bud) Abbott and Lou Costello (born Louis Francis Cristillo) performed together as Abbott and Costello, an American comedy duo whose work on stage, radio, film and television made them the most popular comedy team during the 1940s, as well as a top ten box office draw for a full decade (1942—1952). Their iconic and influential patter routine "Who's on First?"—with its rapid-fire word play and comprehension confusion—set the framework for many of their best-known comedy bits. As a result of its enduring popularity, a video loop of the team performing "Who's on First?" is on permanent exhibit in the National Baseball Hall of Fame and Museum in Cooperstown, New York.
I think this is the greatest comedy skit of all time.