I am not an expert of Holocaust scholarship. But the Holocaust is unprecedented in many ways.
Germany was a normal Western state—the Weimar Republic was in many ways at the forefront of liberalism and governance. It is unprecedented that by legal and popular politics, Germans created the political and lethal machinery to victimize so many other Germans.
The Holocaust was a civil war where innocents were murdered or forced into slavery.
Hitler was a fool. Germans were among the best educated in the world, but Hitler personally was without precedent Europe's worst governor. Nevermind the betrayal of his own people; his economy relied unsustainably on slavery, his theories were bunk, his cowardice was monumental... He benefited from the long-term gains of the Weimar Republic's governance while simultaneously killing or exiling Germany's greatest asset, its intellectuals.
The Holocaust was a vehicle of this, Hitler's personal madness, unlike any other genocide.
Collaboration, most of all, defines the Holocaust. Hitler found collaborators throughout the whole world, from Japan to Moscow. The Soviet Union, itself the most victimized by the war, was for five years allied with Germany. There were Poles who eagerly betrayed their neighbors. Hungarian fascists were condemning civilians to death not months before the end of the war. The French collaborated in the Holocaust. It is unprecedented that anti-Semitism was so pervasive as to infect all of Europe.
The Holocaust showed that Western civilization, not just Hitler's Germany, was eager to execute unprecedented crimes against humanity.
These are but a few ways the Holocaust is distinct from other genocides, from the experience of American slavery, from even North Korean concentration camps and Soviet gulags. It is not about numbers. It is about what our civilization turned out to be capable of.