It's called just Alien.
I'm not sure what the "scariest" movie I've ever seen, there's been a few disturbing ones for sure, but I don't really watch movies for this. The best scary movie I've seen though, is for sure Alien.
It's known as "Alien: The 8th Passenger" in international markets.
Actually the first time i read this. Originally had a German subtitile here, though.
About the topic: The Conjuring is good, The Woman in Black had a great atmosphere as well.
When i was a kid it was pretty much the usual suspects others have named, like Poltergeist or Alien and maybe some b-movies.
Forgot Shining. Again, as a kid though.