This is a tough question. There are so many genres and sub genres of horror.
For sci-fi there's Carpenter's The Thing and Alien, for 'natural' horror there's Jaws; these 3 titles could be classed as horror - or not. The Fog and Night of the Demon are definite contenders, despite the latter being somewhat spoilt for a far too early reveal of the aforementioned 'demon'. Personally I would not have shown it at all, and (just like the MR James' story that it's based on) left the viewer guessing whether the danger was real or just imagined. At the very most I would have revealed it only at the end of the movie.
There are the 'shorts' that are entitled 'Ghost Stories For Christmas' on the BBC, the standout being 'Oh Whistle' (although they are nearly all great adaptations).
I'm going to rule out the horror comedies like Ghostbusters, Ready or Not, Evil Dead and Return of the Living Dead. All great movies, but they aren't true horror movies.
When I first saw it, Blair Witch really impressed (and chilled) me, and even now it stands head and shoulders above other movies of its ilk. Another couple of films that really left an impression after the first viewing were The Exorcist and Halloween; both had soundtracks that really added to the creepy atmosphere. In fact I think that an appropriate soundtrack is one of the key factors when it comes to creating a great horror movie. Another film that left a lasting impression was A Nightmare on Elm Street, although the sequels watered the horror down so that it was more comedic than horrific.
Horror is like comedy; both subjective, so that it's incredibly hard for a director to make a movie that will scare everyone, or make everyone laugh. The danger is that by trying to please everyone, you end up pleasing no-one. Or you go too niche, and make a movie that some are very happy with, but which bombs at the box office.
Worst horror movie? There are so many, most of them being recent (ie last 20 years). Tbh because of the wealth of tv channels, and disc based and internet access I tend to see where most of the poorer movies are going and switch off before the end. Maybe they get better, but I've given them a fair chance after 20-30 minutes.
Most disappointing (I won't say worst) is Event Horizon, which sets itself up to be a great movie. Great premise, soundtrack and visuals - but then it all starts to fall apart. I know that some people really love it, and I've even given it a second chance by watching it again not so long ago; but it still disappoints me as a movie that didn't live up to its potential.
The one that's so bad that it's good? Easy - Night of the Lepus.
Best horror movie of all time? That would have to be The Wicker Man (1973). Just the right length for a horror movie (90 mins) meaning that it can build the intensity up throughout the movie, until arriving at its ultimate, unforgettable, conclusion.
