Views might differ greatly depending on which army people believe to have been most powerful during World War II. But many Asians would respond the same way if the question is regarding the most cruel and inhumane army: The Japanese army.
The Japanese soldiers mistreated POWs (prisoners of war) cruelly and carried out many crimes against citizens including looting, arson, and murder during the war.
While committing countless war crimes, they also purposefully targeted medical staff on the battlefield, clearly disrespecting international law.
Long advocating "bushido education," which honored the idea of "sacrificing oneself for the nation" and saw surrender as dishonorable, the Japanese army had long They thus kept POWs in great contempt, mistreating, humiliating, and killing them.
Furthermore, Japan's logistics system was seriously defective, even trying to feed the POWs let alone supply its own troops.
Some Japanese officials even directed troops to kill sick or weak POWs on demand, so lightening the load. The Geneva Conventions safeguarding POW rights were to the Japanese army only a scrap of paper; incidents of POW abuse and murder were common.