Hitler is today generally considered the worst humanity has ever produced, his name gets used in hyperbolic arguments, and the Nazis are a very popular template for evil regimes in fantasy and sci-fi. Was there anyone who held this distinction in previous eras? Ghengis Khan? Nero? Was there a leader or a regime/dynasty that was popularly considered the worst of the worst long after they were gone?
This is a question that has been asked before – there is always more to say, but you might like to review the relevant section of our FAQ, including an informative earlier response by u/Georgy_K_Zhukov, while you wait for fresh answers to your query.