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