Lots of actor duos achieve status in Hollywood, from crime fighters to criminals, lovers to losers. But few are as enduring as comedy duos. Maybe it's because comedic chemistry is so rare and yet utterly palpable: audiences know it when they see it (as with our list) and also when they don't (Ishtar, anyone?). With the release of Disney's Old Dogs, John Travolta and Robin Williams shoot for the ranks of the funniest ever? and heaven knows the stakes are high.