Alien:
The creature design is amazing, and not just how it looks, the whole lifecycle.
The movies really scared the shit into me when I was a kid and I never forgot about them.
Giger's artwork and designs are truely unique and I just hope we see more of his creative genius realised in film, I have the feeling that his art is the sort which may not be seen again.
No disrespect to Preds though, they are really cool, just not on the same level as the Aliens for me.