Tell me which other poster boy unleashed a world war and decided that genocide was best done in an industrialized fashion? Yes, we have many examples of equally depraved behaviour and wild cruelty by dictators, but few of those unleashed a war that would engulf the world.

Who else put up not only concentration camps, not only labor camps (where people worked to death), but also death camps? Who else, not merely satisfied with these, tried to maximize the hell out of the system to produce as many deaths and as much suffering as possible?

I think the Nazis mastery of propaganda and mass marketing assisted greatly. Nazis wanted to brag about themselves to anyone and everyone they could, making all kinds of stylized films and creating an image of aryan perfection. That isn’t to say Soviets weren’t proficient at propaganda or didn’t make attractive art… as a matter of fact Soviet art can be quite beautiful and modern. But the brand image of “Look how strong and beautiful our pure blooded people are. Don’t you wish you could be just like us?” is more alluring than “Class solidarity” or oddly homerotic paintings of Slavs and Chinese men embracing

He came about at the perfect time: long ago enough that we can compare him to other historical figures like Nero or Torquemada without falling into cultural bias (unlike for instance with Richard Nixon or even Donald Trump). His reign having ended completely long enough not to have any influence on the current world (unlike for instance Mao or Stalin) yet still no so long ago as there are still people around remembering him and his reign (unlike for instance Ghengis Khan)

A lot of the terms in your question are very general and it's not clear what you mean. Sin is pretty clear, but what do you mean by better? You need to put what kind of metric you're using to mean better. If you're just looking at straight up numbers, sure, lots of other leaders have caused the deaths of more people.

But that's not the only thing that's measured when people claim that Hitler was uniquely awful. People look at how much destruction he caused, the time frame in which he did it, Stalin for instance was in power almost 3 times longer than Hitler yet didn't create a European wide catastrophe on par with WWII. Mao was in power for even longer, if you include his leadership of the CCP during the revolution.

You can also look at who the various other contenders killed, and except for Hitler, it was mostly their political enemies, or people who were thought to be enemies. Hitler killed a lot of people just b/c they were there. There's no Maoist or Stalinest equivalent of murdering the majority of the Slavic people for lebensraum. Neither Stalin nor Mao had any philosophy that required the total eradication of groups like Jews and Roma. Stalin and Mao generally killed people b/c of what they did or might do, not b/c of who they were.

You can also look at where they killed. Most of the other contenders were killing people within their own political entities. Hitler rampaged across all of Europe and North Africa, far outside the borders of Germany and Austria.

There's also the impact on what Hitler did to his own countryman. It wasn't just the military that was out killing people. He involved the whole society in the process, from the railroad workers shipping people to camps, to people who were given stolen property of Jewish people, to every business (which was almost every manufacturing concern of any size) that used slave labor. Under Nazism, it wasn't just the military, secret police, and political operatives. It was everyone.