I like Scream because it plays with the conventions of the slasher film (my favorite horror subgenre) so well, and it's an incredibly well written and smart horror film. I also love the references to real life horror films, giving the movie a self aware or meta approach.
#9 Cannibal Holocaust
Cannibal Holocaust started what we now call "found footage" movies (Paranormal Activity, Blair Witch etc.). The movie begins as a straight narrative then become this incredibly disturbing and brutal found footage movie. When it was being filmed the director had the main actors sign an agreement that they wouldn't promote the film for an entire year so as to make their characters fates in the film appear real. He eventual had to go to court and prove the actors were still alive to prevent himself from being jailed.
One of the many films on this list that doesn't need an introduction, but all I can say about it that hasn't been said already is that it still works.
#7 Trick r Treat
Besides #9 this is probably the least known on this list, but one of the best examples of horror anthology ever made.
#6 The Descent
Proof that you don't have to have men in a horror movie to take down the big bad monster. A very very well made movie, claustrophobics should beware.
#5 Dawn of the Dead (1978)
Yes I'm one of those purists that likes the older horror movies way more than the new ones, this one in particular. Instead of just being a zombie movie, Romero took his time in creating a message about people and hiding it inside the film. I had the pleasure of meeting Mr. Romero this past November at a horror convention in Kentucky.
This movie was my first real exposure into horror films, I watched every minute of it the first time I saw it and I ate it up. This movie will always hold a special place in my heart, it created my love for fear and it created one of my favorite slasher killers.
#3 The Exorcist
I got lucky and got to see the directors cut of this movie on the big screen last semester, and it was an incredible experience. It was like seeing it for the first time, since all the other times I had seen it were on television/DVD. Another film that needs no introduction.
#2 The Thing
One of the best (if not the best) remakes of all time. John Carpenter took what was a cheesy 50s sci fi movie and turned it into one of the most gruesome, creative, and iconic horror films ever. If you haven't seen this movie, go buy it now.
#1 The Shining
This needs no intro either, but it is my favorite horror film (and not just because its the only horror film made by my favorite director Stanley Kubrick) but because there are so many levels to this movie that its able to create different scares and meanings for everyone that watches it.