Great to see Session 9 mentioned twice! That is definitely a creepy film. I first got to know about that from someone mentioning it on a forum thread like this years ago. I watched it for the first time with a friend, who at times couldn’t watch the screen because it was creeping him out!
If you haven’t seen Carnival of Souls, which was recommended earlier in the thread, yet that’s another one really worth your time. Watched it for the first time a few years ago and it’s still creepy even for today’s standards.
A lot of the classics have been mentioned already - Don’t Look Now and Rosemary’s Baby especially and with Rosemary’s Baby being recommended, I’ve got to add Repulsion (1965) and The Tenant (1976) to complete Roman Polanski’s “apartment trilogy.”
Repulsion is a masterpiece. No spoilers for this one.
The Tenant is one of the best examples of psychological horror I’ve seen. Roman Polanski plays the main character, who moves into an apartment in Paris where the previous tenant tried to kill herself. He plays a very unassuming man and learns he’s surrounded by neighbours from hell, who complain about everything and accuse him of doing things he hasn’t done… or has he?
Others I rate in the category you’re asking about:
Burnt Offerings. Definitely inspired The Shining as far as I’m concerned. Superb cast - Oliver Reed, Bette Davis and Burgess Meredith are in this one.
The Sentinel (1977). Great cast including Burgess Meredith again! Another film mainly set in an apartment block in America with neighbours, who are not quite what they seem, although in this film they’re pretty nuts from the get-go.
The Watcher in the Woods. This is an overlooked Disney live action film made in the early 80s for older teens/adults. A family with 2 daughters move into a rural manor. Bette Davis plays the elderly neighbour whose daughter Karen disappeared in a chapel in the surrounding woods 30 years ago. It seems Karen is trying to make contact with the daughters.
The Blood on Satan’s Claw. An underrated British horror from the early 70s about a village set in 18th century where the young folk fall under the influence of a demon after a mysterious skull is uncovered. Weird and creepy in equal measure.
The Devil Rides Out. One of the best films Hammer made.