I would say that Hitler and Stalin virtually tie for Most Evil Person Ever. As previously stated, Stalin was responsible for more deaths. We should also pause to remember that if it had not been for his treaty with Nazi Germany, carving up Poland (the Molotov-Ribbentrop Pact), WWII might never even have started, because Hitler would have been faced with fighting a two-front war from the very beginning (in 1939 instead of 1941).
On the other hand, Hitler invented the "Final Solution," the deliberate attempt to exterminate every Jewish person in the world down to the last man, woman, and child - that's a magnitude of evil that I think even Stalin did not quite achieve. True, Stalin sentenced entire peoples to all but certain death, but there was always that slight chance one might escape, or one's children might, because the total extermination of ethnic minorities was not Stalin's ultimate goal - only their complete subjugation to the Soviet system.
So basically we're being asked to choose between two devils, and I guess that here I would follow the example of most Soviet citizens after the Nazi invasion of 1941 - who, much as they hated Stalin, eventually decided that of the two choices, Hitler or Stalin, Stalin was the lesser evil, the lesser devil.