I can see that you all comfortably ignore the obvious.
The Holocaust was the biggest atrocity in history not because the largest number of people were massacred (although 6 million, a third of the Jewish people, is a huge number). It is the worst atrocity because it was planned to details and was industrialised - the Nazi regime established a death industry in order to eliminate all the Jews from the face of the earth.
The Nazis massacred other ethnic groups and nationals but not in a systematic way in order to eliminate them completely just because they were born Jewish - men, women and children.
Although the Nazis treated the Slavic nations as inferior they did not plan to exterminate them like bugs in a similar approach to their treatment to the Jews. The wanted the Slavs to be their slaves in the new German Empire.
Capish?