Alien. Yep, I went there. There is "aged poorly" and "outclassed by basically any half-decent horror film made in the last ten years", and Alien fit squarely into the latter for me. The atmosphere was extremely spotty and the scares weren't anything special. The only thing it did well was dodging some of the worse horror movie cliches. I chalk it up as one of those "you had to watch it when it came out" movies, as it's pretty bad by today's standards.
Apart from that, Paranormal Activity. It blows my mind that people actually think that it's a terrifying series. I want to sit these people down and show them a real horror film like The Others and watch their reaction.