Alien the original move. Why? Directed by Ridley Scott, cast Vern Cartwright should have played the leading role but Sigourney Weaver's dad was a dick and insisted his daughter get the leading role I'm not sure what his involvement was maybe a producer. I have a signed Vern Cartwright photo and her sister Angela (Lost in Space).
I'm not a fan of this genre but Alien did it for me and I loved Jerry Goldsmith's soundtrack which I heard him conduct live and met him the day before.
Tough call between the aliens from the 60's TV series "The Invaders" and the Martians from "Mars Attacks." I'd probably go with the latter, I almost wet myself laughing at the scene where they kept tilting the Washington Monument to different sides to crush the Cub Scouts.