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You're always going to get populist choices (eg. Diana, Princess of Wales; Boy George; John Lennon; Bono) combined with people parading their secondary school history knowledge (Boudicca; Robert the Bruce; King Henry VIII).
It just goes to show that if you're up for these kinds of surveys, you should conduct them every now and again to filter out the dubious candidates, although with characters like Henry VIII appearing, perhaps various notorious tyrants of recent times will get into such "top N" lists a few hundred years from now, especially after various jolly period dramas are made about them, and after everyone gets to dress up in pretty clothes and act out some kind of Hollywood version of events.