Woohoo, one of my favorite genres in film. I noticed that some of the strangest and freakiest movies I've seen are from the 1970s. Also, some have NSFW content, so just a heads-up.
Eraserhead (1977)
A David Lynch film, the one with the infamous, creepy alien baby that never stops crying. It's a black and white nightmare captured on film. Deliciously grotesque and surreal.
House (1977)
Japanese horror comedy that features a schoolgirl spending time over her aunt's house during the summer with her group of friends, and then supernatural chaos ensues. The special effects are so laughably bad, but supposedly it was made like that on purpose. The last 30 minutes of the film are also trippy af.
The Holy Mountain (1973)
A Jodorowsky film that would probably appear normal to those on acid, but for those who watched it sober like I did, it's a strange whirlwind of a film. Lots of mystical symbolism with of course some really interesting weirdness. Also features some hidden enneagram content for those typology enthusiasts.
Belladonna of Sadness (1973)
Psychedelic Japanese anime with a satanic twist. Another 'must be on acid to better understand' movie. Includes a plethora of erotic, psychedelic imagery -- from flying, phallic-shaped demons to hair strand orgies. The animation is also uniquely and gorgeously illustrated like an Art Nouveau storybook -- a la Aubrey Beardsley, Gustav Klimt, and Alphonse Mucha -- something I haven't seen done in film before. It's an interesting duality between insane chaos and ethereal beauty.
Pink Flamingos (1972)
A John Waters film that's as weird as it is atrociously disgusting, so not for the easily squeamish. It's one of those movies you either love or hate, and I personally love it. Along with the crude acts and the acting being so over the top and campy, I still somehow find it all strangely fascinating and at the same, morbidly hilarious.
Fritz the Cat (1972)
Ralph Bakshi's very first film and the first animated X-rated movie at the time. It's actually a really funny and pretty thought-provoking movie though if you get beyond the weird animal orgies and crude humor -- the kind that makes fun of everyone and everything, so wouldn't be a good film for those who are easily offended.
Invocation of My Demon Brother (1969)
A Kenneth Anger short film featuring a bunch of hippies smoking pot and engaging in a satanic ritual for a dead cat. Includes lots of rich, psychedelic and occult imagery with no dialogue. It also features an annoyingly repetitive musical score composed by Mick Jagger with a synthesizer, but it makes the film that much more creepy, imo.
Un Chien Andalou (An Andalusian Dog) (1929)
Lastly, one from the late 1920s -- a silent short film created by Luis Buñuel and Salvador Dali. It's basically Dali's art coming to life on film. A bizarre, surrealistic masterpiece and very well executed for its time, too.