Horror I've given 3.5-stars or better; look at the list notes to see individual ratings. I also had personal criteria on what qualifies as horror for me, e.g. Alien is, Aliens is not; Evil Dead 2 is, Army of Darkness is not.
Edit April 2020: changed the cutoff from 3 to 3.5 stars, and updated for the first time in a few years.
Also my Top-25 List is here, but if you like horror at all you've likely seen them all already.