Great bad guys make for great science fiction. Think of the Daleks, the Cybermen and the Borg. I'm sticking with the big screen for this category though and also sticking with yesterday's film. There is something really creepy about the Reavers from Firefly and Serenity. As Zoe put it in one of the TV episodes:
"If they take the ship, they'll rape us to death, eat our flesh, and sew our skins into their clothing - and if we're very very lucky, they'll do it in that order."
The film provides an explanation for their behaviour which was a mystery in the TV series, and it somehow made them even worse. Honestly, Reavers could give me nightmares if I let them.