Well, I am not sure that I would have selected your guys
The obvious #1 is what happened to Hitler? Some believe he could have escaped.
He’s dead (he would be 127 year this year) : )
More seriously, although circumstances of his supposed death preclude any certainties there is little doubt if according to Occam's razor (Among multiple hypotheses, the more likely is the one with the fewest number of assumptions).
When comparing the usual story (surrounded, he suicide himself in his bunker and the other members of the bunker decided to burn the body - either by respect or by fear of coming USSR soldiers) to the "run and hide" hypothesis (in an invaded country, manage to reach a submarine incognito, to flee to argentina or elsewhere, manage to stay keep a low profile for his remaining life, please note that in this hypothesis Jukov and SMERSH were purely honest with the allies as regard to their doubts) there is no doubt for me, the second one is highly improbable.
Why did the world let Hitler build a Spartan country in 1933...
ie... School children learning how to take apart rifles as part of the normal schooling, etc.
Europeans were sure that he was crazy but were also convinced he will erase Bolchevism? Bad assumptions, bad luck…
No for me the greatest mystery does not involved unknown fact but reaction impossible to understand: how could European and US have reacted as they did during the SS. Saint-Louis Odyssey?