There isn't much to say about Alien that hasn't already been said.
42 years on, and it is still the finest example of sci-fi horror ever put to film. For me, this film does not age - the spectacular special effects are still effective and terrifying to this day.
The cast does an exceptional job of portraying every day life of a space crew of regular people. They're not soldiers, nor are they super heroes. Their reactions to what happens are very believable.
An absolute masterpiece of cinema.