Cannibal Ferox, The Howling sequels (don't really want to though), most of the stephen king adaptation I have avoided like the plague because I don't want them to shatter the images his books have left in my head.
Salo (correct title p.b), is more of an exploitation film than a horror. Probably the most depressing film you will ever see, its difficult to recommend something so horrific, but you should see it because it is a powerful piece of work.
hmm....I've seen quite a bit of horror movies but here are a few I just recently have seen:
Amityville Horror
The Changeling
Children Of The Corn
...and here are the ones I haven't seen...
The Fog
From Hell
28 Days Later
Day of the Dead
Last House on the Left
...I'm sure there are others I haven't seen but I can't think of any more right now.