Hitler 11-14 million dead / Stalin 15-20 million dead.
Personally I see no real difference between them; both were paranoid tyrants.
But in sheer numbers of his own people who died under his rule, Stalin takes the the lead as most undesirable figure of the 20th century for me.
I still find these numbers hard to comprehend in any reasonably tangible fashion. The mind boggles at such cold inhuman and calculatingly precise events of this kind...