Skull. simply because it begs the question why the f**k does it have a human skull underneath a translucent gel cowl. Everything about the Alien is set up to be some scientifically accurate (well relatively speaking) parasitic animal, and yet it still has that one completely unsettling unexplainable aspect to it. I think the almost supernatural element on top of a well thought out biological creature works. Just something eerie about the mixture between science and the supernatural to me. At least that's how I always took it. It can be argued, well blah blah it carries DNA and so there's a skull blah blah that's no fun, and it obviously wasn't Giger's intention. The fact it's an actual human skull only adds to it.