There are so many listed here that I love. Oblique Strategies nailed most of them... I would be hard pressed to make a top 5 as I think what you enjoy or want to watch at any given time is dependant on your current state of mind and mood, much like listening to music.
I was pleased to see Zardoz... and Dark Star! What a classic!
Dark Star was written by Dan O'Bannon who was also behind the script for Alien, there are actually quite a few precursor ideas that materialised in Alien (minus the slap-stick feel) in Dark Star. The concept of teaching a smart bomb phenomenology being included in a film's plot... still makes me laugh, sheer brilliance!
Has anyone mentioned "
Outland"? I loved that film, very much a Western set in space, but also very much a classic sci-fi film. Those who listed Moon - check out Outland,
Moon has references to that film, (along with homage to a few other sic-fi classics).
I will put my vote in for
Alien over Aliens. The original was the epitome of brilliant edge of your seat, suspense-thriller, schlock-horror science fiction. James Cameron's sequel was great fun, but like most things James Cameron does was as subtle as a sledge-hammer. Speaking of James Cameron,
The Abyss deserves a mention in this list.
Quatermass and the Pit scared the hell out of me when I was a kid,
almost as much as Alien did when I saw it at the cinema when it first came out...before anyone knew what the Alien actually looked like (a big part of the suspense in the film, not knowing what the Alien actually was or looked like)