I'm not sure whether you mean what others think, or what you think? From your list, I'd probably have to go w/Imperial Japan, based upon fun stuff like the Rape of Nanking and the Bataan Death March. Especially given that the Japanese continue to deny or at least downplay what happened in Nanking to this day. 'Course you can say the same thing about Turkey and what they did to Armenians (and to a lesser extent Greeks.) Especially the denial bit.
However, If I could add my own choice, I'd probably say Cambodia under Pol Pot. Not only were the Khmer Rouge mass murderers, they were insane nutjob mass murderers. As in, having to wear glasses was enough to get you labeled a "counterrevolutionary," and a bullet in the head.
_________________
"The man who has fed the chicken every day throughout its life at last wrings its neck instead, showing that more refined views as to the uniformity of nature would have been useful to the chicken." ? Bertrand Russell