For me, the Alien is the best. I like the Alien for a year before I even saw the movie. I had seen the Kenner toy and I owned a jigsaw puzzle. I knew the story from reading the Heavy Metal graphic novel at the PX. To me, the creature design was so original. The only thing interesting about the Pred is his head.
Now if we're talking if the Alien and Pred were real, I'd have to go Pred. Why? The technology obviously.