As the topic says, discuss, debate here about it. Even argue your choice ! I'll start.
Big Mass Effect fan here, and since all my cartoons, movies, games that I experienced, I never found someone or something else more powerful, evil, twisted and somehow terrifying than the Reapers.
If they were looking like space spiders, even more terrifying (thank god no) but the ability to be impenetrable, only to a combine massive fleet to barely scratch you, the ability to influence the course of entire civilizations and even control the minds of all organic beings... well that's quite an evil brilliance there.
Rudimentary creatures of blood and flesh, you touch my mind, fumbling in ignorance, incapable of understanding. - Sovereign
The most evil group is in my opinion the Sith, with all their groups (all empires and the First Order). Just because they have their superweapons (that Sith-temple on that planet, the Deathstar and the Starkiller Base), they use them to kill a lot of innocent people. That is evil.
Hagen from Tronje because killing Siegfried was necessary and killing someone who believes he is invulnerable by cunning is just what needs to happen to "the ultra-good" in epic stories. Can't stand heroes that are too white-washed. They need to meet their end because of being too arrogant in their sincerety.
Interestingly enough, I really love villains that have a lot of backstory or perspective twist to it.
Handsome Jack from Borderlands comes to mind. He's the villain from the player's perspective in such a brilliant way, but in reality all he's doing is trying to bring civilisation to a lawless, backstabbing bandit planet before taking things personally after spoiler stuff.