OP implies he/she has seen most of the more common thrillers, so this may not help. The first Halloween is one of my all-time favorite true horror movies - and the one all the others ripped off.
“Saw” (a.k.a. Texas Chainsaw Massacre) good horror, but diluted with a little too much camp in the dinner table scene.
If you want a more modern, under-rated film, you might want to check out Salem’s Lot, a TV movie starring David Soul, believe it or not.
A very good horror film.
For campy horror, you just can’t beat Evil Dead 2, which is really a remake of the original Evil Dead except with better production values. The sequel, Army of Darkness is a lot of fun, but not really scary.
The Wolf Man starring Lon Chaney, The Howling, and An American Werewolf in London are the class acts of the werewolf movies.
I also found Pet Sematary to be very frightening. Good Steven King adaptation.