Penn doesn't direct a ton of films - only 4 in the last 19 years - but every time he does, he puts out work that is heaped with praise. His output as an actor might be a little more hit and miss, but he's still got 2 Best Actor Oscars to his credit.
Eastwood has been stepping behind the camera since 1971, crafting films in a wide variety of genres and often directing himself. With a string of well received films, not to mention 2 Oscars for Best Picture and 2 for Best Director, his name has become synonymous with prestige.
One of the greatest and most lasting stars ever to grace the screen, not to mention a master craftsman. His films are timeless classics and his character The Tramp is one of the most recognizable in film history.
The yin to Chaplin's yang. Keaton was not as successful as Chaplin, but his work is just as enduring. He was, quite simply, a brilliant performer and director.
Could there be any other? Few actors have ever been able to match the screen presence that Welles had and few artists can match the skill he showed behind the camera. Not all his films are great - he's infamous for having made some films solely for the money that he would then use to fund a pet project - but even his less stellar films have some redeeming quality.