Adult horror: Pet Sematary, The Shining, Firestarter, Cujo ... basically everything but Tommyknockers.
Stephen King is known as the best for a reason. His newer stuff can sometimes veer more toward the philosophical than the out-and-out spooky, so if you want to be scared pants-less, check out older stuff like Pet Sematary.
Just don't watch the movies. (And yes, I am including The Shining in that.) Stephen King short stories and serials make for some killer movies (The Green Mile, The Shawshank Redemption, Stand By Me) but for some reason the horror just doesn't seem to translate well.
Young adult horror: The Waking Dark
I read this story last year and I've thought about it, on-and-off, ever since. A small town is shattered by a horrific event--twelve people murdered on the same day, killed by people who then killed themselves. The story is told from the alternative perspectives of five teenagers who were witnesses to the murders that day as the town slowly devolves into paranoia and madness.
Young adult paranormal: The Coldest Girl in Coldtown
This one is for those who only like a little bit of scary, mixed with a healthy dash of romance. Set in an alternative America where vampirism is an infection that is quarantined in "cold towns," this book follows teenage Tana as she tries to help get her infected ex-boyfriend to a cold town before he completes his transition and kills her. The only person who is willing and able to help her? Gavriel, a vampire Tana discovers tied up in a farmhouse where most of her friends have just been killed. I've re-read it twice since I've gotten it, and believe me, you want to check it out.
And last but not least, Jezebel's annual "spooky story" selection. For the last 3-4 years, Jezebel has had commenters leave their scariest stories in the comments section and selected a handful of the best for readers to shudder over on Halloween. Scroll through--there are a few years' worth to choose from and I promise you won't be disappointed.