I've been recently getting into the Italian giallo genre. They were mainly a thing from the mid-60s to mid-late 80s, with the early-mid 70s being when they were most prominent. Had mystery elements that were inspired by Agatha Christie or Hitchcock, but with the focus on stalking-n-killing that became an inspiration for the 80s slashers like Friday the 13th or Halloween. And since they're Italian, you also have plots that don't necessarily make sense, as well as a fair bit of sleazy sexuality -- to the degree that a few films have cuts where legit porn scenes were inserted into the movie.
I'd watched Mario Bava's Bay of Blood/Twitch of the Death Nerve a few years ago because I'd read that Friday the 13th II had openly stolen a couple of its kills, but just recently watched his Blood and Black Lace, which was good enough to make me dive down this hole. The direction in a lot of the films is just amazing. Beautiful settings with great camera work that both shows off the scenery and also builds suspense. Ennio Morricone (most known for his work on Clint's spaghetti westerns) does soundtracks for a lot of them. Just great stuff.