Alien
Just the best ever. The final half hour is pure magic. Will always be one of my favorites of all time, and fairly easily. If Letterboxd had a top 5 instead of a top 4, you would likely see it there.
And yes, and much as I love both of them (they are both essentially perfect), it is better than ALIENS, to me by a decent margin. I enjoy the vibes of ALIEN much more, and generally is more captivating to me. Also, Scott in his prime is a better director than Cameron. Sorry!
Never gets old to rewatch. Ever. Anytime, anyplace, and day.