Facehuggers are the only thing in Alien and Predator that scare me. I actually panic a tiny bit every time I see them. Especially once the psycho-sexual design motif was pointed out. I hate hate hate the idea of having something violate me like that. So claustrophobic and painful.
I have tonic clonic seizures sometimes, and let me tell you, having your body move on its own with you unable to resist is the most horrifying experience I've ever had. Not only is it physically painful, but it's psychological torture to your sense of bodily autonomy. Plus there's the very real fear of dying every time. Pair that with rape and forced impregnation and you have real nightmare fuel for me.
Of course, all that fear is what draws me to things like Alien. I live for fear.