I don't know much about the rest so I say Hitler or Stalin.
Hitler
His racist theories were irrational and he knew it. That's for sure. There's this video I saw about "how Nazi Germany lost the war", near the end of the movie they showed what the allies found in Nazi Germany. "Worse then Genghis Han and the Mongols," they said. Now that's pretty bad. Though I don't believe it since the Mongols killed over 50 million Chinese people (half China's population at the time) alone, and who knows what he did to Russia, Poland, Iran, Iraq, Ukraine, White Russia, Korea, etc. So Hitler killed 6 million jews, committed numerous crimes against the German people, lost the war (I really believe the Germans could have won), and killed almost 30 million people outside Germany's territory one way or another. But these actions Nazi Germany took were not cause by Hitler alone, you must understand. As soon as 1921, A certain type of psychologists called the "Jungists" started publishing articles about a Blonde Beast stirring in the unconcious of the German mass shortly after the first World War. Hitler found millions of willing tools and people.
Stalin
He simply killed too many people. By the 1930's Stalin had already starved off like 6 milliion Ukrainians, had already beaten Hitler's massacre of the 6 million Jews. What pisses me off most was how their army raped 15 million women (every woman) aged 10-80 in German territory when they marched in nearing the end of World War II.
Mao
I guess he's not very popular but he could be appropriately considered the biggest mass murderer in history. Under his rule, more people were murdered then those in the Soviet Union or Nazi Germany.