A difficult choice.
I'd have to say Stalin. While the consensus is that Mao killed more people than Stalin or Hitler, more or less with programs such as the Great Leap forward that led to a lot of deaths the problem wasn't malicious intent like the deaths caused under Stalin or Hitler, but stupidity. As someone pointed out to me the other day, Mao actually believed a lot of the stuff he said, while Hitler and Stalin were just evil.
As for Hitler he was evil and all but
1. Stalin almost defiantly killed more people
2. While Hitler's rule certainly had long-lasting implications after he was gone, because of Stalin we got the cold war, which shaped US foreign policy till the early 1990's, leading to such things as the Arms Race, McCartyhism, etc.