I've been on a bit of an Alien/Aliens kick lately thanks in part to an interesting message board
thread on the Something Awful forums. The thread began as an analysis of James Cameron's
Aliens and how it was the original poster's opinion that it could very well be one of the greatest movies of all time. Without succumbing to too much hyperbole I'll go out on a limb here and say that the franchise on the whole is one of the greatest and most iconic sci-fi/horror franchises ever. In fact, it is my opinion that
Alien is one of the scariest movies of all time. This post is about why that is.
Before we proceed I'll just go ahead and let you all know that I don't claim any of the ideas I discuss here as original. Mostly I'm just summarizing what was discussed in the forum thread.

Blow it out the goddamn airlock!
Let's take a look at
Alien, the origin of the franchise. What was it about this film that terrified audiences when it came out? Giger's Alien, that's what! Look at that thing. (In fact it's probably because you don't ever actually get a good look at it that it's even more terrifying). Is it made of metal? Does it have eyes?? It is otherworldly to say the least. In fact I would posit that the Alien is less of a creature grounded by reality and more a demon from the blackest corner of space. Space Hell if you will. The Alien does not eat, it does not sleep, it feels no compassion. As Ash put it in the film, the Alien is "unclouded by conscience, remorse, or delusions of morality". It exists for a singular purpose: to rape and murder everyone.
Wait, what?
That's right, rape. Alien is terrifying because it is a movie about being horribly raped and murdered and it plays up this fear by taking the typical victim of rape, the female, and making her (Ripley) the only member of the crew who isn't. This movie subverts our preconceived ideas about sexual violence, horrifying and fascinating us in the process.

Curiosity killed the cat.
Let's start with the setup. The crew of the Nostromo wake up in the middle of the galaxy because they intercepted a signal of some kind coming from some uninhabitable rock designated LV-426. They set down and investigate. They find an alien spacecraft that appears to have been there for years. Note that Kane, Dallas and Lambert enter the derelict craft through a large opening that looks suspiciously like a giant vagina.
Alien is full of weird sexual themes and imagery so bear with me. There's more to come.
Once inside, Kane (played by John Hurt), lowers himself into a vast cavern populated by leathery "eggs or something". He gets up close to one to investigate. Suddenly, an Alien facehugger erupts from the egg and attaches itself to Kane's face, forcing a tube down his throat and literally subjecting him to oral rape.
So Dallas and Lambert bring Kane's unconscious body back to the Nostromo and the infirmary where they attempt to remove the creature but quickly find out that it's impossible without probably killing Kane in the process. Just as a rapist will be more likely to murder an uncooperative victim, so too will the facehugger finish it's task or kill the unlucky host.

Somebody get some Pepto-Bismol!
Before long, the creature removes itself from Kane and he wakes up, seemingly none too worse for wear. Which brings us to the most iconic scene in the film. Enjoying one last meal before returning to their cryogenic slumber for the home trip, Kane begins to act a little odd. And by a little odd I mean thrash around violently before a small alien creature explodes from his chest. You could say that Kane was reverse raped by his own child which itself was the product of rape.
It's no strange coincidence that the full grown Alien is itself basically a giant walking penis. Just look at its head. It doesn't get much more obvious than that, people. So anyways, what does this giant walking penis do? It sneaks around in the dark, jumps on its victims when they least expect it, and forcefully penetrates them with its second mouth which fires out of its bigger mouth with enough force to kill.
It goes on like this for the remainder of the film as slowly the crew is whittled away until all that's left is Ripley and her cat Jones. While it's tempting, I won't go into the implications presented by the fact that the alien is wholly uninterested in the cat. I'm sure you can draw your own conclusions there.
There are plenty of other themes in
Alien that I'm ignoring in this post. Maybe I'll talk about them some other time. Really though, Alien is one of the scariest movies ever made because whether you realize it or not, it toys with your fear of being raped and murdered in a horrible fashion.
Alien is one of my favorite movies ever. It's too bad garbage like Alien: Resurrection and AvP had to get made. I like to pretend they don't exist.