Well, D.B. Cooper was himself. What happened to him and all that money is a mystery.
The Babushka lady has come forward and I've seen her interviewed in various Kennedy Assassination videos on Youtube.
The man in the iron mask may be a fictional character, although there was supposedly a prisoner who was forced to wear a mask over his face. It is generally supposed he resembled the King just a little too closely.
V for Vendetta is a fictional character. Although various outlaw groups now use that mask.
Jack The Ripper may have been any of a number of men who have been identified by researchers, if you read the works of any of these researchers it tilts towards their suspect. In the strictest sense we have a line-up of suspects, but don't actually know which one committed the murders.
My choice would have been Who Buried the Treasure on Oak Island, and why?
_________________
I wake to sleep, and take my waking slow. I feel my fate in what I cannot fear. I learn by going where I have to go. ~Theodore Roethke