Well, for cinematic horror and suspence, Aliens was the ultimate - the soundtrack was brilliant, the idea of having to face it all nearly alone with nowhere to run (another great device - anything that you simply CANNOT escape is brilliant - Evil Dead showed that) was great. I realize it wasn't intended to be a horror per se, but it puts you on teh edge of your seat and keeps you there - which I think is the whole point. Saw Pitch Black recently and saw some really nice similarities.

Jacob's Ladder was intense and very acid-esque. Not sure I'd qualify it as a horror. Brainfuck might be more appropriate. More like Eraserhead than a horror, to my mind.