The Great Dictator (1940), a satire on Nazi Germany, was Chaplin's greatest commercial success, but it has aged badly. There are only a few really funny scenes, the whole hangs badly together, and - most of all - the Nazi's perpetrated such terrible crimes that it now seems irresponsible to turn them into buffoons. That is of course hindsight, but history is history.
That Chaplin had the same piece of dirt (some call it a mustache) below his nose as Hitler, had of course been noted and Chaplin uses the "resemblance" in this film. He plays both dictator "Adenoid Hynkel" and his double, a poor Jewish barber, who suffers from amnesia because of WWI and when he finally returns home finds the ghetto were he used to live the hunting ground of Nazi bullies. The film intercuts between the persecuted barber and the ego-maniacal Hynkel and only brings these two strands together in the last 15 minutes when the film finally becomes the expected "mistaken identity" tale.
Some funny scenes are the ballet between Hynkel and a balloon globe, or the competition between Hynkel and "Benzino Napaloni" aka Mussolini, played by Jack Oakie in a very strong performance (such as cranking up their barber's chairs when they have a shave together, to sit higher than the other). The scenes where Chaplin plays the Jewish barber are much less interesting due to their mawkishness, although we have Paulette Goddard as Chaplin's girlfriend. The best sequence is a syncopated shave Chaplin gives a customer to the tune of one of the Hungarian Dances by Brahms, all in one take.
But other scenes in my view go wrong. For example, while Chaplin enacts a good copy of Hitler's speech mannerisms, by using only silly German words like "Schnitzel" and "Sauerkraut" he turns the threatening figure of the dictator too much into a lightweight fun character.
As has often been noted, the weakest spot of the film is its ending. Chaplin, speaking as the Jewish barber who is impersonating Hynkel, suddenly becomes Chaplin himself and holds an impassioned plea to end tyranny, obviously forgetting that satire is a sharper weapon than mere sermonizing. If direct address would be sufficient, film (art) would not be necessary anymore.
Concluding: on the positive side, Chaplin in this film had the courage and the foresight to attack the Nazi's, but on the negative side they turned out to be real evil and evil is not something to lampoon. In the end, The Great Dictator is too soft to fit the crime.