Britain was the primary responsible party for Dresden I believe. The blantent intent of killing mass amounts of civilians ranks pretty high on the atrocity list.
Germany has too damn many to count, but between the Holocaust, the Einsatztruppen and other such things, they handily outdid everyone involved. Of course, many of the Soviet Unions atrocities got "handed off" to Germany and I don't know if we'll ever have a true accounting for them.
True, the rape of Nanking was not an official operation, but it was probably a planned series of attrocities. The Japanese DID do a lot of official operations that directly led to the mass slaughter of millions of Chinese. I'm not completely certain whether history classifies those as "unofficial operations" (which would be completely ridiculous), but consider how well modern day Japan admits they ever did anything uncivilized in WW2. The death camps that the US POW's were subjected to were light by comparison contrasted with what happened to the Chinese.