People can multiply answers such as this ad infinitum
--based on the population in the affected area, one could allege that the slaughter inflicted by Boudicca in first century Britannia was the worst in history.
The point about the NSDAP is the scale, the organization and the exportation of their genocide. The Turks did not export the killing of Armenians. The Hutus of Rwanda slaughtered Tutsis there, and have done so in the DRC--but no formal organization for the slaughter was erected as was done by the NSDAP, and there was on conscious "exportation" of the slaughter by a Rwandan government. Pol Pot's horrors cannot necessarily even be described as genocide, at least to the extent that that was not the conscious aim--he had some sort of horribly distorted plan à la
Mao's cultural revolution.
People who are unfamiliar with the NSDAP plan, and who can stomach the implications, should search for "Wannsee Conference." This was the meeting early in 1942 during which the "final solution" was promulgated. Lines of responsibility and authority were set up and formalized. A chain of command was established within the NSDAP and within the SD-Sipo (SD was the Sicherheitsdienst, the security service of the SS and the NSDAP). SD was given responsibility for security in "ausland" ghettos, and in the concentration camps. The responsibility and accountability of the Gestapo and all the agencies of the SS were detailed, and the relationship of the army to the SD was established, making the army subordinate to the SD in all "security" considerations in the "ausland" (this resulted in hundreds of instances of SD officers commandeering Wehrmacht supplies and/or transport, often for use in the ausland concentration camps, or to evacuate death camp inmates and SD personnel as the Soviets advanced).
The "final solution" was organized in a meticulous manner to which it is arguable that no other NSDAP or merely German enterprise was in the last three and a half years of the war. Before the Wannsee conference, certainly there were concentration camps, and the Nazis were rounding up Jews, Slavs, Gypsies, homosexuals and the physically and mentally disabled. But the Wannsee conference established a bureaucratic organization to accomplish the purposes of the final solution, which was arguably the most well-organized and most powerful bureaucracy in Germany.
There is, quite simply, nothing else in history which compares to the intentional, organized genocide perpetrated by the NSDAP, which exported that genocide and the mechanism to achieve it in the wake of Germany's military invasions.
There is nothing in history which even remotely resembles what was done by the NSDAP.
(I wasn't picking on you, Dashenka, i was just using your post as a point of departure.)