I saw IT when I was about 10 years old. That stuck with me for a while. IT is definitely the young me answer.
I had seen it all by the time I was a teenager. As an adult, the few movies that stick out are already mentioned loads in the thread. A Serbian movie and Martyrs.
One of the things I find most unsettling in horror movies, when done well, is being powerless and trapped inside of your own body. For that reason, even tho it’s not an amazing film, but a decent one, The Skeleton Key comes to mind. And I enjoy The Serpent and the Rainbow for similar reasons.