Aliens for me. In my fantasies the aliens don't use humand for sex, just as pets. The feeling that I'm an enferior being-a pet, is highly erotic to me. Like the idea of aliens forcing a male human and female human to mate for study, to bread, turns me on.

As for vampires, eh, they're great in the movies, just never really got the sexual attraction to them. Then again, that's just my personal preference.