Alien and Aliens
Can't choose between them. Two different but excellent movies.
Alien 3
Liked it at the cinema, but it really came to life for me when I saw the Assembly Cut. The story makes more sense, the characters feel more real, and it has a great mixture of epic and claustrophobic scenes.
Alien:Resurrection
A pointless addition to the series, but I suppose an enjoyable piece of scifi action. I still watch it from time to time