For me it would be a tough decision between Hitler and Stalin.
Hitler loved kids and animals, as do I, and instituted some of the most progressive animal cruelty laws in the world for the time. Stalin seemed so intelligent and self-aware of his evil that it is incredibly interesting, for example: "The death of one man is a tragedy, the death of one million is a statistic."
*The title is of course very subjective, in the end any person you consider to have done significantly "evil" acts will suffice - could be anything from a serial killer to a politician.