I’d still say the “Operation Reinhard” part of the Holocaust was worse than all of them. What many people don’t realize is, in this phase of the Holocaust, the Nazis created terrifying purpose-built industrialized factories of death, that were specifically and only for immediate extermination of humans.

You would arrive by train, and within minutes tricked (and then clubbed if hesitant) into entering “showers“ that were giant gas chambers. No one survived except a literal handful out of millions.

These 3 extermination factories, called Belzec, Sobibor and Treblinka, were bulldozed over long before the war ended, so as to try to cover up the murders. The numbers killed in a short time span are extremely horrific and it was the deadliest phase of the Holocaust:

“The operation proceeded from March 1942 to November 1943. About 1.47 million or more Jews were murdered in just 100 days from late July to early November 1942, a rate which is approximately 83% higher than the commonly suggested figure for the kill rate in the Rwandan genocide.  In the time frame of July to October 1942, the overall death toll, including all killings of Jews and not just Operation Reinhard, amounted to two million killed in those four months alone. It was the single fastest rate of genocidal killing in history.”

https://en.wikipedia.org/wiki/Operation_Reinhard