First some commentary on the thread:
For those insisting that the OP needs to provide an objective definition of ‘evil’, get real. The real world doesn’t have an objective definition of evil; it is, as has been noted, in the eye of the beholder. Nobody thinks they are the villain. This is obviously an opinion thread.
Now no civilization is completely good or evil, but neither is every person. I’m sure Hitler was nice to his mother, at some point in his life he probably helped an old person across the street and helped a friend move furniture. But one can take a person and sum them up as ‘good’ or ‘evil’ in an overall sense, based on which was dominant. Obviously whatever good Hitler did in life is massively overshadowed by what evil he did, so it’s uncontestable he was an evil person. So in the same way I think it’s fair to categorize civilizations as good or evil, while doing so in no way says they were solely such.
Now to actually respond to the OP:
First choice goes to the Mongols. Any society that wracks up a kill count comparable to twentieth-century regimes using medieval technology and out of a much smaller world population clearly has some issues. The population of China in 1400 was HALF of what is was two centuries earlier (not all of which can be attributed to Mongols-the Black Death was during this period as well-but if the Mongols are responsible for the death of a ‘mere’ 10% of the Chinese population they are responsible for as many dead as the Final Solution). There is a tale that the Mongols seriously considered exterminating the north Chinese population and turning the whole area into pasture. That it was even considered is damning. Genghis Khan’s answer to the question ‘what is best in life’ is that of a psychopath.
Second choice is the British. Partly this is for their global reach; they were assholes all over the planet in ways that few ever were. There is a lot of condemnation of colonialism and imperialism yet what often gets left unsaid is that the British Empire was the premier exemplar of colonialism and imperialism. They oppressed and subjugated more of the world and more people than any other state in history with the exception of the Mongols. They may have done nothing individually worse than what other powers were doing at the time, but they did more of it.
Now I can already here some protesting that the British also gave us some nice things too. I’m sure it brought warmth to a Bengali’s heart that although his children were starving because he’d been put out of work by Lancashire textile imports, the wealth the British were sucking out of India was helping to create modern democracy (in which he could not participate because [insert racial expletive]).
Seriously, this is the state that decided that Chinese efforts to stop the British illegally smuggling opium into their country was a casus belli. This is the state that literally invented the term ‘jingoism’.
Part of my decision for picking the British is that there is still a great deal of defense for it. This forum has a serious problem with fanboyism and apologism for the British Empire. There are lots of claims that the British Empire wasn’t a giant extortion racket, that the British built lots of good things so that somehow justifies it. In this respect the Mongols were better. At least they were honest. “I want your stuff, so I’m going to kill you and take it.” Brutal, but there isn’t the gag-inducing self-righteous hypocrisy that one gets reading about the 19th century British empire.
And for those arguing that a civilization shouldn’t be labeled ‘evil’ because it did nice and/or shiny things with the stuff it extorted from others, that is one of the premier arguments for imperialism. ‘The natives can’t manage their resources wisely; we should ‘help’ them be more ‘efficient’.’ If one murders a man, even though he take all the victim’s wealth and give it to orphans, he is still a murderer.