Pearl Harbor to Hiroshima/Nagasaki is not a useful comparison. Far more relevant would be comparing Hiroshima/Nagasaki to what the Japanese did to civilians in China, Korea, Southeast Asia, the South Pacific, etc.
Every major player involved in World War II committed at least some war crimes, even the Americans. But between the Nazi war criminals and their Japanese counterparts, it's pretty much a tossup about who was worst of all. I'll say "Nazis, but only by the slimmest of margins."
Also, the Japanese have the mitigating circumstance of never having been all that civilized a nation to begin with. With the benefit of hindsight, one could say that if Commodore Perry and the European trading powers had just left the Japanese alone in the 1850s, let the Tokugawa shogunate keep up its isolation policy, the world would have been a lot better off. Under the Meiji Restoration and the subsequent "shogunate of the Army" (in all but name,) Japan became less civilized than they had been under the Tokugawas.
"I don't deduce, I observe."