Personally, my reason for liking Alien more than Aliens is entirely based on aesthetics and personal taste. I think they're equal in quality, but Alien hits a certain spot for me, the cinematography, the set design, the slowly increasing tension. To this day, it's still one of the scariest films I've ever seen due to the steady build and the fact the intensity never really drops. The chase scene near the end is my favorite example,
Spoiler
even if Ripley's not actually being chased
, the way it's shot, the camerawork, the sound design, everything is tense and terrifying all the way to the end. I'm also more of a horror kind of guy in general, though I LOVE Aliens a LOT, I'm less of an action-horror guy and prefer pure, unfiltered terror to thrills. Both are still 10/10's from me, though.