Well I've now watched the first 10 films, I must admit that generally, I've been quite disappointed. Let's see how Alien faired against my ridiculous ideals...
Considering it was made in 1979, the realism in the Alien creature is quite astonishing. On top of this, the writers have devised an ingenious way to make the story have a great deal of validity and raise many issues.
Sigourney Weaver plays the lead role as Ripley, third in command on a freight [space]ship struggling to gain the respect of the men working under her while her superiors are off the craft. It tackles the issue of humanity's reliance on technology - a man who turns out to be a military robot is jeopardising the lives of all aboard because Alien holds such military potential. Lastly, the whole situation arose from what could be a "right to life" debate - should the Alien's host be left to die in quarantine, or do they hope to save him and risk the lives of all aboard (the choice made is obvious because there would otherwise have been no film).
The creature of the alien itself is very well thought out. The first sighting of it is as a parasite attached to a crew member's face. This parasite is in fact not killing him, but supplying him with oxygen whilst also laying an egg in his throat. This parasite cannot be removed because all of the Aliens' blood is "concentrated molecular acid". The parasite eventually falls off and dies and the new host is left feeling normal.
This instills a clever "calm-before-the-storm" feeling within the film right before the metaphorical defecation collides with the metaphorical air circulation.
Anyhow, the chest of this now content man explodes and frees an alien that grows within minutes from a rat to an eight-foot monster by living on nothing but spacedust aboard the ship... The biggest downfall of this story.
The ending of the film isn't particularly clever either. An extra 2-3 minutes are added on purely to show the tiny pants / tight white shirt / errect nipples combo. Sigourney Weaver pulls it off quite well, but it's still relatively pointless.
Overall, not a bad film.
Alien on IMDB