We should throw Pol Pot in there too coming in third by numbers alone. Though if you say Hitler started World War 2, then his death count is between 36 and 40 million.

Hitler tends to be seen as the worst because though it is one thing to be killed because you spoke out against the government, it's another to be killed just because your grandma was a Jew. Hitler killed to wipe out and annihilate on the basis of race, Stalin/Pol pot killed because they was paranoid and because believed it would better their country in their own warped way. Hitler built the world's first industrial death factories. Plus he is looked down by the U.S. because he fought a bloody war with the United States and our allies, whereas Stalin killed millions of his political enemies. Hitler stills has a following (looks at some people on this BB), Stalin has few, if any, followers still.

Oh yea, and Nazis make better film villains.

Really, though, what difference does it make? The end result is the same: corpses. All three were profoundly evil people and just great role models for murderous megalomaniacal tyrants!