Alien is the only movie that makes me want to go back in time to experience it on the big screen. I would probably have peed in my pants watching it but it would have been totally worth it ;).
There are a lost of western that can be named, that´s for sure. The ones I mentioned are my favorites, because they are a lot of fun.

My father saw it when it came out - he said it was genuinely the scariest movie he'd ever seen. In a good way.

McLintok sounds like a movie right up my alley. Looking forward to it.
I have to rewatch Alien as soon as possible, it´s just been to long since I have watched it the last time. In 1979 the tension must have been unbearable, not knowing what is roaming the Nostromo. Such an awesome movie.

@Susanne: I envy your father, even though it was scary. But I´m glad that he kind of enjoyed that scary experience :).

I wasn't allowed to go - my parents didn't think I was old enough. (They were quite right.)

I don´t know how old you were at the time but I guess you parents were right. I watched "The Silence of the Lambs" when I was 12 or 13 years old and I couldn´t sleep all night. Talk about a nightmare inducing movie.

Oooh, Midnight Lace and Some Like It Hot.
Great list altogether.

Alien and Aliens still remain in my favorite movies :)

I'm not a big Western fan, but Some Like It Hot might have made my list if you didn't mention it. I think of it whenever I wear my really high heels.