I've watched a lot of horror movies, which means that most "classic" horror tropes doesn't work on me. The only two movies that has genuinely scared me as an adult are A Tale Of Two Sisters and The Babadook.
What these two movies share is that there aren't any jumpscares and that we never ever really get to see the monster, my mind seems to make up way scarier stuff when it doesn't get to see the monster.
A Tale Of Two Sisters: This movie scared me so bad that I had to pause in the middle of it and relax, the tension is so high all the time and there's never any release, really.
Plot from IMDB: Two sisters who, after spending time in a mental institution, return to the home of their father and cruel stepmother. Once there, in addition to dealing with their stepmother's obsessive and unbalanced ways, an interfering ghost also affects their recovery.
The Babadook: Same here, not many jumpscares, just masterfully crafted tension, the whole movie felt like sitting on needles.
Plot from IMDB: A single mother and her child fall into a deep well of paranoia when an eerie children's book titled "Mister Babadook" manifests in their home.