Alien or Aliens?

I vote Aliens. I still think James Cameron made the best film in the franchise.

It is just impossible to compare them for me. They're completely different films, with totally different goals, both succeed massively, and stand amongst the very best movies in their respective genres.

Scott's movie has the sort of ingenuity of concept, they're not soldiers, so they have no real weapons, even if they did they're fighting a creature with acid for blood on a spaceship, couple that with the god tier production design, and it's tough to beat.

Still, Cameron's movie is a thrill ride for the ages, with no less wonderful aesthetics in its favour, and the supremely strong character arc at its centre that lends the whole film a much more human element in spite of its larger scope.

In short, the only vote I'd cast in such a comparison is one to break a tie

Alien is king. Hell, I personally prefer the Assembly Cut of Alien 3 over Aliens, but that's just because of the performances and the depth it gives to Ripley's character. Aliens wins out over that one on a technical level, though. And Alien: Isolation deserves to be mentioned in the same breath as these films.