Make it a two-part answer: (1) old school and (2) modern.

For me, old school horror is dominated by two names: Lovecraft and Stoker. If you haven't read them, I highly recommend you give them a shot. Just be forewarned: they're wordy. But they're such absolute masters that all those words spin a web that ensnares the reader.

I don't read a lot of modern horror. Most of what I read is modern takes on the Cthulhu mythos. But I think I may look to expand my horizons in this regard. Your recommendations may point me in promising directions.