Alien
I watched it in Film Studies (which is the best subject ever) along with 'JAWS' in preparation for a comparison essay. I've watched JAWS once before, but never 'Alien'. And I prefer 'Alien'.
Sure, when you compare the gore and horror to recent R Rated ketchup-fests, its not really that scary. It has the classic elements of Horror, and I did jump a couple of times (THAT DARN CAT!!!) but today's audiences, like 'JAWS', will find most of the effects humorous. Yeah, back in 1979 people would have been vomiting in the isles, but whilst the infamous 'dinner table scene' would have been stomach churning - today it is actually really funny.
The only nitpick I have is that the only character I ever cared about was Jonesey the Cat...apart from when that stinking fur-ball keeps making you jump! But apart from that its just adorable. Its really funny when Ripley is running away from the Alien (which is BAD A$$!!) but is taking the Cat along with her. In many ways, the cat adds to the movie, so I dont think Scott should have left it out. But....who brings a cat in space? (Awaits lolcat image....)
Oh, and the behind the scenes details are more stomach churning than the special effects. Seriously, a guy had to have sheep testicles stuck to his face?