Ok, serious answer. The reason why Nazis are considered worst in history is not because they came first in a "who killed the most" competition of some sort, but because:
1. Nazis rose to power and murdered millions because democracy failed.
Unlike communist China or Russia the Nazis weren't brought up by a violent uprising or hateful mob usurping power. It wasn't "you either with us or you're dead" scenario - it was a direct effect of democracy. It was the will of minority and passiveness of the majority of people that gave Nazis permit to atrocities. Many people later commented on it being the first example of civic passiveness (including the famous quote of a protestant pastor: "First they came for the Socialists, and I did not speak out. Because I was not a Socialist." on so on).
When majority of people don't find a party that represents them, a well organised minority can rise to power (pay attention UKIP voters). At the peak of their support in 1932, NSDAP enjoyed only 33.09% of support at 80% of electorate, which means that only 1 in 4 Germans actively wanted them in power. In fact, before burning the Reichstag down and comandeering the absolute power by force, the Nazis were already loosing support and politically were on the way out - in presidential elections 1932 Hitler lost to Hindenburg 37 to 53%. But firm support of just 1/4 of the nation while the rest of the electorate was splintered among multiple opposition parties or on the fence, was enough. Proportional Representation system gave them majority of seats in Reichstag, they voted in all the powers they needed and everything else is history.
2. Nazis used philosophy, science and technology to excuse and to perform their deeds.
Yes, Mao and Stalin also performed mass atrocities inside their countries in the name of political classes or theories and used "the greater good of the fatherdom" as an excuse and yes, it is true that especially Russia had a long history of active anti semitism, but to be absolutely frank - nobody in 20th century was particularly surprised by wild east countries like China or Soviet Russia going psycho or "peasant revolutions" going off the rails. Even nowadays, by modern standards, the news that China would violate human rights of its citizens or fact that Russia attacks Ukraine or bombs one of its own suburbia states is not THAT much of a surprise to anyone, really. It's sick, but we don't really expect much of Russia - it's run by a guy who's either President or Prime Minister ever since we were born, he rides horses topless and sleeps with bears in winter, the country itself seems very brutal and violent - even a minor road rage scene can easily turn into gun shootout or baseball bat match - looking at what crazy russkies do is almost like weekend youtube entertainment to us.
Meanwhile, even in 1930ies, for a leading, central European country of honourable Prussian Generals, elite scientists, philosophers, theorist and doctors (before World War II, Germany had generated more Nobel laureates than any other nation) to turn around on the spot, flick the switch and gas few millions of civilians while the crowd is cheering, vivating and sieg heiling was a scenario nobody could believe or phantom.
Yet Nazis did exactly that, they employed philosophical model to mass murder "undesirables" using the betterment of humanity as an excuse . As in - "they are lesser species, the world needs to be cleansed of them, they need to be annihilated". It wasn't a tribe against tribe. It wasn't irreparable difference of political opinion and direction. It wasn't enemy combatant they pointed at. It wasn't an army of well-off neighbouring country they selected for public enemy. It was literally "if you don't like ******, now is your moment" permit for purge. They pointed at the individuals the crowd did not want in their neighbourhood - the people with darker eyes and hair running corner shops, people in the wheel chairs that couldn't fend for themselves and begged on the streets, at divergents - gays and trannies, at homeless sleeping in doorways, those annoying socialists handing leaflets on street corners, the creepy smiling preachers that knock on your door on Sunday to "talk about God" and stuff - they pointed at them all and said - "there's your problem, you have this non human element among you, remove them from society and you'll be better off". And the rest of the Germany, the first world, civilised, educated, European, modern society looked at what they were pointing at and nodded - "all right, if you say so".
3. The timescale and volume
When we talk about soviet crimes, crimes of Stalinism etc, we talk about decades. Decades of messy, dirty, internal secret partisan war fought one denunciation, one individual, one group at a time. The "total solution" took merely months. The first Auschwitz I opened its gates to mass transports in May 1940. In November 1944, by the order of Himmler gassing operations ceased across the entire Reich. During 4 years 6 months 15 to 20 million civilian non-combatants lost their lives in process, on industrial scale never encountered before or since.
It's not just about pure numbers, it's about clockwork meat grinder like efficiency and volume of the process - it was methodically organised, mathematically computed, technologically innovated, industrially developed, built for purpose and theatrically arranged for increased efficiency annihilation. En masse gassing of everyone who wasn't usable. Women. Children. Elderly. Handicapped. Weak. Alike. Zyklon B in "showers". While their belongings and teeth were recycled for precious metals. Ordered, documented, paperwork approved, calculated from the time scheduled railway throughput per day on pan-European level all the way down to "shower" barrack capacity per hour and layering techniques improving square mileage of mass graves. Nothing was ever late, nothing was not done, nobody went on strike or complained to their supervisor about workload in the process. Those that survived selection were either utilised to death or medically experimented on (including kids).
And all of that because... them aliens. Because "this land is ours" and they come to take what's ours. Because lip trembling, flag saluting, teary eye "let's make Germany The Great Reich Again". Because the family living next door or running corner shop looked different. Spoke in tongues. They walk like we do. They talk like we do. But they're not one of us. It happened here. Next door.