It is clear that the nukes in Japan are the biggest war crime in the history of Earth. Pearl Harbor was a theater conducted by FDR. The attack was against civilian population only and radioactivity damages still remain a major problem.

Number two is the Iraq war - this has happened in our times when most people should have had access to reliable information and right to honest governments. More than 5 million people have been slaughtered and 3-4 million are refugees. The whole population was very close to death because of this ethnic cleansing which started 1980 when USA ordered Saddam to destroy Iran with US assistance. The latest aggressions took place in an independent country against the resolution of the UN. This war crime involves extensive and brutal torture and assassinations comparable to holocaust. The 911 was used as a fake excuse. Israel is also highly involved.

Number three is the Vietnam war and related mass murders in the neighboring countries done by USA (Kissinger main war criminal). Estimations vary very much but could be 10 million civilians. The war started as a result of the Gulf of Tonkin conspiracy where US marines were shooting their own ships and accused Vietnamese.

Number four should be Afghanistan. The country has suffered war for 50 years and the whole population is suffering and millions dead.

Number five is Hitler's holocaust - Wikipedia on holocaust deaths: This gives a total of over 3.8 million; of these, 80–90% were estimated to be Jews. The ethnic cleansing was systematic like in the M and N East wars (USA and Israel major war criminals).

By the way, there are academic investigations about the war crimes. I may provide links whenever I will find them next time.