Obviously, if someone asks now to think of the most hated/cruel man to live, most would respond with Hitler (in my own opinion). Before this, who was seen as the "Hitler" of the past?
Answers to this will differ by culture, throughout time, but you can get started on some previous responses in the FAQ
It varied from place to place. In fact, it does so now. Hitler being regarded as the embodiment of evil is very much part of Western culture, but it's not really part of non-Western cultures.
With regards to figures despised across the whole of Western civilization in the same that the Nazis are now, the only thing which really came close were the villains of the Bible (the Pharaoh of Exodus, Judas, Pontius Pilate, etc.) and the Roman emperor Nero (who was known for persecuting Christians). Since Christianity was essentially the only thing which united all of Western civilization, it only makes sense that its universal villains would be the figures who look most evil from a Christian perspective.