I know one could call it Sci fi but I think Alien was number 1 in horror for me...in terms of pure dark atmosphere, creepiness, and even chains and water dripping as well as the sounds of salivary dribbling; Alien was number 1 for me.
“Here kitty....meow...here kitty...what ...arrggggh!”
I still remember the dolby surround theater I saw it in with chains just clinking off into the sonic distance or the self destruct activation sequence...”The ships self destruct system has been activated...you have thirty minutes to reach minimum safe distance...with a blurr of lights and rushes of venting steam and various alarms...and the creepy alien peering into the box where the cat had been stored....
Being cheap at the time, I was reading the paperback of “Alien” instead of seeing the movie. It was thoroughly creeping me out. But when I got to the part where Ripley killed the android, I put the book down, went to the piggy bank for funding and left for the theater to see the movie right away.
A GREAT plot, which few people understood after seeing the movie. It is one of the best monster movies ever.