Just to be a bit retentive, the movies cited, for the most part fit into what I call Terror movies. I am from the old school, where horror comes from the inside. Many of these movies do not allow that, but force it upon you, terrorizing the viewer.
Classic movies such as Frankenstein (1931), Dracula (1931), The Woflman (1941), The Mummy (1932), etc. are horror movies to me. Horror should include conflicts of humanity and the individual against the unnatural. IMHO.
Personally, I can't stand terror. However, of those movies below cited, I think Aliens was one of the scariest movies I ever saw. I saw it in the theatres when it first came out. I wouldn't watch any of the others in the series until they were on TV. (My 11 year old thinks the Alien series is great, and made me take him to Alien v Predator.)
Of the "Dead"movies, I like "Night" best. It was so campy it was enjoyable.
I remember seeing the Exorcist in the theatres, in a second release in 1974. Except for one or two moments in the film, I found it funny. Maybe it was that I was a junior in HS.
I agree that "Lifeforce" was a really good Sci-Fi movie, very interesting premise, but not what I consider the greatest. "My all time favorite Sci-Fi is "Forbidden Planet". On that note, of the three genres I note here, I'll take Sci-Fi.