Alien is as perfect as a film can get. The ensemble character-actor cast is one of the best ever assembled, it's impeccably paced, gorgeously shot, boasts an amazing and atmospheric score, has a simple story turned intriguing because of all the underlying content and thematic material, and displays incredible set design, costume design and practical effects. It's the whole package. Not only is it a masterpiece of production design, but is one of the best films in the genre to use its deliberately paced, slow-burn buildup as a way to induce concentrated terror. The gore plays second fiddle to effective buildup and suspense (unlike most horror films), the likes of which are handled brilliantly.
On the design of the film, its artists are also one of the best teams ever put together, with a concept art team consisting of H.R. Giger (creating perhaps the most memorable and enigmatic movie monster in history) Jean "Moebius" Giraud, Ron Cobb and Chris Foss; a team just as responsible for the film's undeniable legacy as director Ridley Scott himself.
A cultural and filmic Icon, Alien is one of Horror and Sci-Fi's undisputed greats and will live on in infamy as one of the most innovative films ever made.