Hitler. Even though Stalin and Mao killed numerically more people, most of those deaths were either the byproduct of a shortsighted economic plan or were punishment for dissent. The Nazis killed because people were born into the wrong race. From my point of view, there is a sort of rational limitation to the atrocities of Stalin and Mao in that, if you were loyal or at least indifferent to their regimes, you'd generally be left alone. There was nothing that a Jew could possibly do in order to avoid death other than somehow cheat the system. Hitler was also ultimately unsuccessful in his plan: if he had managed to conquer the Soviet Union and beat back the British and Americans, not only virtually all Jews (upwards of six million) but also tens of millions of Slavs would have been killed in an orgy of violence that would make Stalin's purges, Mao's Cultural Revolution and potentially even the famines the two communist rulers caused look tame in comparison.
Baghdadi follows close behind since ISIS' ideology is equally genocidal, although its emphasis is religious rather than racial, so its victims - primarily the Shia, but other unprotected religions as well - would at least have some possibility of saving themselves by converting. Pol Pot would also be a close second, because although his genocidal ideology was racial, it was at least contained within the borders of Cambodia rather than implemented on a continental scale.
