I definitely would have to go with Alien (1979) the film has amazing atmosphere and the creature effects are outstanding. I also absolutely love the chemistry between all of the characters they feel like real people and not just some script. Not to say Aliens isn't as amazing as the first but the first movie just got so much right between the claustrophobic tension built in those small isolated corridors. and by far my favorite scene is the death (or capture if you watch the directors cut) of Captain Dallas. That scene still scares me to this day. Although Aliens does bring alot to the tables, with its memorable quotes and amazing set pieces and just overwhelming tension brought by the fact that so many xenos are on screen really brings both of these films as high masterpieces to say the least.
Predator is also amazing I really love the first one but two is good too.
