Who committed worse war crimes? The Nazis against the Russian people, military and civilians? Or the Japanese against the Chinese, military and civilians? Do try to keep it limited to this.
In this comparison numbers don't matter. Only methods and reasons matter.
Only Russians, no Jews, Poles, Greeks or Eastern European nationals? Seems a strange question really, leaving out 6 million people. Since I have been to the .... of Nanking Memorial (a large underground museum) in Nanjing, I would have to say the Japanese.
They managed to convert humans into logs of wood, at least in their own minds. Froze arms and then shattered them off, then the leg and so on to see how long the men could stay alive without limbs. They made both a ritual and a competition out of decapitation, while the nazis shot their prisoners in the back of the head. Sadly many of the most ruthless Japanese war criminals who claimed scientific protection were given reprieves in return for their horrendous findings. There are many more cruelties that I won't go into but when I was growing up my mother told me that propaganda around that time had the Japanese soldiers bayonetting Chinese babies and throwing them into the air. She taught me not to believe in propaganda, but when I went to Nanjing last year I found evidence that it was true. So I went the full circle of denying to myself that the Japanese did such things, to seeing (along with so much other) evidence of the same atrocities exposed during the 1940s.
Amazing what the odd Toyota plant set up in your state will do to the truth.