One thing I notice about my list (100-76; 75-51), or at least think I notice, is that I'm biased towards newer movies. I don't necessarily think I like newer horror movies more than old ones, but I do now believe that my scoring system, being weighted heaviest on the scariness factor, naturally biases my list towards the present. By and large, horror movies have become scarier. This is in part due to audiences having a stronger stomach for horror movies over time, and studios engaged in a race to the top in aspects like gore, shock, and raw horror.
A good example of this is the movie Oculus (2013). I am surprised to see it this high on my list. I think it's an underrated horror movie, but I would not have guessed it was my 30th favorite horror movie of all time. But the movie is DAMN scary, and really serves to make you feel dreadful throughout most of the film. Event Horizon (1997) is a movie I'm plum surprised to see on my list at all. It's not a very good movie, but as a horror vehicle, it is scary, visually appealing, and the story is so confusing it ends up working somehow.
When all is said and done with this list, I'm likely to fool around with my weighting system, and see what types of lists are generated from them. This one is surprising.