That's pretty open ended. There could be a healthy debate among the many worthy candidates: Churchill, Ghandi, Mandela, Reagan, the Roosevelts (Teddy, FDR and Eleanor) ...
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William and Mary? They were good people. I don't know...
This question, of course, is entirely a matter of opinion. Many would say that, at least in Europe, Adolf Hitler was the bad guy of the 20th century. Encouraging the rise of Nazism and sending many Jews to concentration camps, he advocated the expansion of the Aryan race and condemned those who were different to exile from society and, sometimes, even death.
The similarities between the 20th and 21st centuries lie in innovations. The 20th and 21st centuries have been periods of great strides in technology and other areas.
Radio was not an important invention in the early 20th century. Instead it was developed by Heinrich Hertz in the 18th century. The cotton gin (APEX)