there's some real interesting stuff in the Alien Series documentary, which might be on the dvd (I saw it on television): several scripts for Alien 3 were rejected, including one written by William Gibson; Ridley was originally written to be a male character, and when they changed it to a woman they didn't alter any of the dialogue, they just had a secretary change all the pronouns; there were several interesting scenes in the first two movies that were deleted; Sigourney Weaver was angry that they deleted Ripley's daughter; Michael Biehn was angry that they killed Hicks, but Weaver thought it worked; Ridley Scott filmed his children in space-suits to make the exterior of the Nostromo look bigger. it's definitely worth a couple of hours if you're a fan.
as to the All-Time Greatest tag, I have to muddy the waters and nominate Blade Runner and Terminator 2: Judgement Day (hmm, there's Ridley Scott and James Cameron again).