It's crazy to me that they were able to create a creature as original and terrifying as the "Alien", especially for 1979! This was way before CGI and modern technology could create monsters like this, and they did it with practical effects!
shareIt was good exactly because they didn't have CGI. They had to come up with something that looked genuinely alien even though it was always going to be a man in a rubber suit. So everything was about taking that human shape and making it look inhuman. The actor was over 7' tall but weighed about 140 pounds, so he didn't look human, and then the elongated skull, the lack of obvious eyes and the pipes growing out of the back also made it look inhuman in shape.
Add drool and slime and you're away.
It's effective because it takes human shapes but distorts them with mutated human genitalia. The alien head is phallic, as is its protruding mini-mouth that forcibly penetrates skulls and brains (and a tail that seems to have raped Lambert), the chest-burster is like a big toothed cock erupting from your chest, the face-hugger is a human hand with vaginal centre that contains, yet again, a tube that forces its way into your mouth and plants a seed inside you.
Geiger took the concept of sexual violence and crafted a creature that embodied it. Biological nightmares of involuntary (male) birth, mutation (the Dallas-egg), and rape are perhaps the most elemental and powerful types of nightmare, and are called forth as we watch this hideous specimen essentially fúck the crew to death.
It deserve its place as a horror icon, alongside the vagina-mouthed Predator, but that's another story...
AvPgalaxy.net has some pics of unused concept art from Prometheus (and now rumored to perhaps be seen in Alien:Covenant) of the neomorph or ultramorph. A potential result of the SJs/Engineers experiments with the black goo.
Of course its all speculation at this point whether or not this will be the Xenomorph creature or one of the Xenomorph creatures we will get in SRS's upcoming AC.