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Without a doubt, it was the Japanese. First, all sides tended to treat different groups or nationalities differently. So the Germans treated Brits and Americans better than they did their Russian POWs. The Russians were cruel towards Germans they captured (especially SS) but were lethal to all of Vlasov's White Russians they captured (and they were also pretty bad to Russians who they freed from German POW camps). Approximately 30% of Allied (British, Australian, US, and Dutch) POWs died in the hands of the Japanese during WW2. But it was worse if you were Chinese and military.

The Japanese captured over 2 million Chinese troops and guerrillas during the war. They returned less than 100 alive at the end of the war. That is a mortality rate of 99%. There are concentration camps run by the Germans who didn't have mortality rates as high as this. And the Japanese did things like...using Chinese prisoners for bayonet practice, holding a beheading contest (two Japanese officers had a race to see who could be the first to behead 100 Chinese POWs), eating body parts like human liver, and burying them alive. And this does not include the human experimentation done on Chinese (don't know if those were POWs or captured civilians) at Unit-731.