Of the Silver Age was undoubtedly Adolf Hitler. Of course he mostly appeared in World War II comics like Sgt Fury and Our Army at War, but there were several occasions where he popped up in the more modern world.
In this story from Blackhawk #115 (Aug 1957), a series of daring attacks sends a rumor around the world: Hitler is alive! However, it turns out to be a hoax as a gang of crooks have plotted to steal the Nazis' hidden treasure which is in the hands of a fanatical Hitler supporter.
Hitler made a memorable appearance in Adventure #314. A criminal manages to evade the Legion's elaborate security system and steal their only time-bubble. He heads back into the past to gather some of the greatest villains of history: Nero, Dillinger and Hitler. When they come to the present he manages to switch the minds of those three villains into the bodies of Mon-El, Ultra Boy and Superboy respectively. But Saturn Girl senses that the villains can be defeated by informing each of the weakness of the others (Superboy to Kryptonite, Mon-El to lead, Ultra-Boy to radiation). Sure enough they all kayo each other and their minds are transferred back into their evil bodies:
Der Fuehrer also popped up in Jimmy Olsen #86. Jimmy is shocked when watching an old WWII film to discover that he had a double on Hitler's staff and decides to go back in time to investigate. He arrives on D-Day and sees Eisenhower on the beach (a goof, since Eisenhower did not travel to Normandy until the day after D-Day). Jimmy convinces the Germans he's one of them, and after some amazing predictions of his come true (all learned from the history books), is rapidly promoted up the ladder to the German High Command. He was that German general he saw in that film.
Unfortunately, Jimmy neglects to tell Hitler about the plot on the latter's life by the German generals and he is about to be killed when the time travel mechanism he used (a Professor Potter "time bomb") returns him to the present.
In Fantastic Four #21, Stretcho and the gang battle the Hate Monger, a man who is stirring up hatred in the US and a revolution in South America. In the end, the villain is killed by his own supporters and revealed: