Ooh, some toughies.
1) Gotta be The Excorcist.
2) Hellraiser
3} King Kong: you gotta respect the classics, especially ones that still hold up more than 70 years later!
4) Jason and the Argonauts: see above. And, Ray Harryhausen.
5) Total Recall: one of Schwarzeneggers best (not that that's saying alot...)
6) Star Wars: I want to buck the trend, but Star Wars contributed so much to the modern day SF genre, it's hard to discount it. Plus, Lucas wasn't an a**hat back then.
7) Highlander
8) Hmm; toughest one of them all, IMO, but I'm gonna back ET. Probably because I was so young, but it filled me with a sense of wonder that I haven't seen before or since. Alien is a great horror/SF movie, but I've seen dozens since that are just as good.