Les Diaboliques (1955)
Christina is a long-suffering wife, paying for her husband's affairs, as well as their mutual business, a boarding school. She has had enough and rather than divorcing him, she decides to kill him with his mistress, Nichole. That's a fine beginning for a Hitchcockian film, and it is enough to get me enthusiastic about where this film is going.
Frankly, if this had been a Hitchcock film, I wouldn't have been surprised. It is a Gothic, a thriller, a puzzle, a psychological mystery, and it is a lot of fun. The direction is excellent, as is the acting and cinematography. Frankly, the only weakness I find in this film is calling it "horror". There is a scene or two that one might label influenced by that genre, but it is misleading to call it that, let alone put on a "best of horror" list.
Still, I highly recommend it. Sandy, if you were thinking of adding a very "safe for Sandy" film that is listed as one of the great horror films, this would be a good one. I think it's right up your alley.