The best alien movies of all time, in no particular order, are:
It wouldn't be an alien film list without Ridley Scott's horrific vision of that place where no-one can hear you scream. The titular creature – a Freudian nightmare designed by HR Giger – was never so terrifying in the sequels as when it stalks the unfortunate crew of the Nostromo.
Fun fact: the role of Ripley was originally written as a man. That scene in which Sigourney Weaver wanders around in her pants at the end wouldn't have had quite the same effect on audiences as a hairy bloke in Y-fronts.
Ridley Scott will be trying to top Alien with his prequel, Prometheus – coming out in June.