TCM aired a really bizarre Senegalese film called “Xala” a while back.
And here’s Roger Ebert’s opinion. (He rated “Pink Flamingos” zero stars, stating simply, "How do you rate a movie like this?)
Yet another bizarre movie I’ve seen that hasn’t been mentioned yet is a Greek film called “Dogtooth.”
The movie is unrated, but would be NC-17 for explicit sexual content.
Third vote for Liquid Sky.
The Human Centipede must be way up there but I’ve never seen it.
I mentioned *Dogtooth *earlier in this thread after Colibri mentioned The Lobster by the same director:
I watch a lot of weird movies and have seen a large number listed here and will be watching a number I haven’t seen.
My top choices are already taken so I’m adding two that I quite enjoy a lot but don’t rise to top of the list for weirdness though both are weird.
My choices “Man Bites Dog” and “Otis” both are dark comedies. Neither I believe are mainstream but Man Bites Dog may count as one.
I own the soundtrack
Sorry; too many weird movies to remember.
I also saw “Liquid Sky”; I told my brother about it, and he told me that he saw it in a theater, and halfway through, the theater had to be cleared because there was a tornado warning, and they offered refunds and most people took them, including him and his friend.
A Methed-Up Fever Dream if I ever had one! Made me question everything Sean Connery ever did before or after.
And, every thing David Lynch ever did.
Aside from the premise, the first film was a fairly conventional horror movie.
The sequels, on the other hand…
Eternal Sunshine of the Spotless Mind
The film class in college had the professor smugly showing this, and even the stoners in class were going Da Fuq???
Honorable mention to all the B, C and D movies that I had to project in the 1980’s. My personable belief is the Friday the 13th movies were all the same, with the reel going in different orders…
When the Wind Blows - strange animated film about an elderly couple preparing for, experiencing, and eventually dying in a nuclear holocaust. Bizarre just because of its subject matter and simple, totally straightforward plot. There are zero twists or surprises, but it’s completely emotionally devastating by the end.
ETA: If you haven’t seen it, it’s still worth watching even though I completely just spoiled it. 
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It was based on a graphic novel of the same name. Like many of the warnings of nuclear apocalypse that came out about the same time it’s devastating, and shows the outmoded thinking of many involved (in the case, the old couple, who think of war in terms of WWII).
Andy Warhol’s Frankenstein is pretty far out. I haven’t seen it since 1991 or so but I thought it was crazy when I did see it.
It was great in 3D.
I can’t believe that Kevin Smith’s Tusk, hasn’t been mentioned yet. Stars Justin Long as a podcaster that ultimately gets kidnapped and over time transformed into a walrus. 'Nuf said.
Last Year at Marienbad
I mentioned this one back near the start of the thread, but it’s good to see that a.) somebody else saw it, and b.) thought it was weird.
Typecasting. Such a curse.
Just remembered this one - a totally bizarre German film called “71 Fragments of a Chronology of Chance.” It was…intense, I guess.