The strangest movie I’ve actually watched in its entirety is Cube, a science fiction/horror film released in 1997 about a group of people trapped in a maze of cube-shaped rooms.
Doctor Strange
It has 3.7 Imdb points — I didn’t realize their scale went that low. I’m tempted to watch it just to what such a bad but strange movie is about! 
I’ve seen Forbidden Zone at least 3-4 times and have always enjoyed it. Taken as homage to the ethos of Fleischer Studio cartoons of the 1930s, it’s not even all that strange.
I’ve also seen Possession – on a Betamax tape! – and found it not so much strange as awful and virtually unwatchable. The same director (Andrzej Zulawski) made L’amour braque (1985).
I have tried to watch it twice, getting no more than 30 minutes in either time. The first time, I thought I was the victim of a horrendously bad English translation. Upon further investigation, I discovered this was not accurate; almost every line of dialogue in the film is a non-sequitur or nonsense. And it’s “based” on a Dostoevsky novel!
Horrors of Malformed Men (1969)
Not a cheesy horror film, but a dysfunctional family drama, emphasis on dysfunctional. One of very few movies I have ever seen to truly capture a dream-like state of existence.
Erendira (1983)
Based on the novel by Gabriel García Márquez, who also wrote the screenplay, this is a very watchable piece of Magical Realism.
Marquis (1989)
Sometimes disgusting flick has actors in minimally expressive animal suits playing out a 19th century Bastille-based story centering on the Marquis - a dog with a talking penis (voiced by a woman) - among other perverse faux-fauna. Notwithstanding the gimmick, it’s intelligent and entertaining for discriminating viewers.
Toto Who Lived Twice (1998)
I watched 7.5 minutes in all. The first scene was some guy having sex with a cow. I take it for granted the rest was strange as well.
***Curdled
Harold and Maude
Succubus***
Astronaut is also my choice for strangest film.
While there are weirder films, I think the works of Ken Russel have been sadly ignored in this thread, especially as they were intended to be relatively “mainstream” films, at least for their (very strange) period. I’d suggest Lizstomania, a biopic (in the broadest sense of the word) of 19th Century composer Franz Liszt, starring Roger Daltrey, a giant prosthetic penis, Nazis and Ringo Starr as the Pope.
The Lobster and Dogtooth (also by Lanthimos) were the first movies that came to mind.
Also:
Mulholland Drive by David Lynch
Oldboy (the original Korean film by Park Chan-wook)
Skidoo.
I mean, I always wanted to see Jackie Gleason trip out on acid…
:eek:
What, Leonard Maltin never saw Staying Alive?
1967 - The Day The Fish Came Out with Candice Bergen.
I think I came across it in the mid 70’s on PBS. Haven’t been able to find it on streaming services.
A possibility for the strangest but biggest box-officed ones I’ve seen is Yellow Submarine.
Speaking of Terry Gilliam, there is Tideland, which is pretty controversial, but I like it. (trailer)
Similar in that it stars a young girl is Jan Švankmajer’s Alice, a mix of stop-motion and live-action and based on Alice in Wonderland. (trailer)
Similar in being stop-motion is Blood Tea and Red String. (trailer)
I especially like Naisu no Mori/Funky Forest (whole damn movie–watch it) And by the same director Cha no Aji/The Taste of Tea. (trailer) (fan-made trailer)
Japan has plenty of candidates for strangest, though–for instance, Big Man Japan. (trailer) (movie)
I see from the wikipedia page that Leonard Maltin agreed with me.
“there is not enough peyote in the entire American Southwest to render this movie comprehensible or endurable”
Streets of Fire: Streets of Fire - Wikipedia
It’s just really odd. Most of it is like they started something and then gave up. I don’t mean because it’s multi-genre; that is probably the underlying reason, but multi-genre films can work well. This felt like everyone on a student project being given a camera and a cast and then someone put it all together at the last minute. The overall feel is really, really weird.
What? No love for Upstream Color?
I saw Time Bandits bombed out of my mind, which is really the only way to watch it.
What, no mention of Freaks ?
And 7 Faces of Dr. Lao, with Tony Randall playing seven odd roles.
And then there’s The 5,000 Fingers of Dr. T.
Tie between Satyricon and The Holy Mountain.
Hmm I could make a long list, but you’ll be able to google for similar lists. Off the top of my head, picking ones obscure enough I don’t remember others talking about them…
Wax or the Discovery of Bees in Television (something like that) about a worker in some sort of airforce flight simulator installation who gets mentally obsessed or possessed with some sort of bee archetype
Drawing Restraint 9, a thematic conclusion of sorts to abstract things artist Matthew Barney (?) does art about, starting him and Bjork as lovers who conduct a ritualistic tea ceremony on a whaling ship which turns them into sea creatures. You have to be able to enjoy a slow hypnotic but kinda trippy pace, but if you do, it’s fascinating.