Nazi Germany, hands down. I know this sounds ethnocentric, but 20th Century Germany was a center of modern western civilization, it claimed to be an heir to Christian love and humanistic enlightenment, and went in two generations from being one of the most liberal and tolerent European nations regarding its Jewish citizens to being the architect of the most horrible atrocities of modern times. Only Stalin's Russia comes close; Stalinism because it also stemmed from a 20th century western base in a culture which should have had more respect for human life. It's nowhere near as bad, however, because by all measures, early 20th century Russia was nowhere near as westernized and influenced by enlightenment values as Germany. My third worst would be the Inquisition because these mass murders were performed by people believing they were Christians perverting a religion which teaches tolerence and unconditional love. We need to realize, however, that Spanish society and culture of the 1400-1500's had been informed by years of religious war between Christians and Moslems, and was a pre-enlightment, much more brutish culture.
With the possible exception of North Korea and modern Islamic radicalism (if that's what "integralism" means), none of the other candidates represented cultures which would be expected to be aware of or practice respect for individual human lives, as further tempered by modern political notions of human equality and human dignity. Certainly, the acts these cultures and groups can be considered "evil" in an abstract sense, but this "evil" was being practiced by people who did not know what they were doing was "evil". The fact that the Nazis attemped to hide their holocaust and Germans tried to pretend it wasn't happening shows that they were fully aware of the import of their crimes.
Regarding those who didn't make the "10 most evil list", I wonder about the absence of Maoist China, the Cambodia of Pol Pot, and even Revolutionary France, which by many measures were far more "evil" than the Ottomon Turks, who over most of their history were fairly tolerant by the standards of the time (occasional mass murders of Armenians and others notwithstanding).