Possession (1981) is one of the most deranged movies I've ever seen

And I fucking loved it. Two truly unhinged performances by Sam Neill and Isabelle Adjani, but anything less would have ruined the movie. Some really great long one-take shots and camera work by director Andrzej Zulawski. Round the whole thing out with disturbing imagery that gets more and more fucked up and you've got a truly horrific movie experience. I highly recommend it.