Zabriskie Point (1970) great sound track, weird movie. Late 60’s Riots on campus. Acid Trip deluxe
Anything by Andy Warhol
Anyone else think Oldboy should make the list?
The Forbidden Zone is off the friggin hook! A must see for any Danny Elfman fan.
I’ll second Requiem for a Dream, and then I’ll toss out THE #1 MOST FUCKED UP MOVIE OF ALL TIME…
The Star Wars Christmas special. I mean seriously, WTF?
Check out Island of Death. I rented this because I thought the description I read must be an exaggeration. Nope. The fact that the two main characters are siblings is the least appalling thing in the story. A turning point early in the movie, the point at which have the audience will turn away and decide to give the rest of this saga a miss, is when the male lead, in a beautiful Greek-island resort setting, abruptly starts humping the bejesus out of an adorable little baby goat. After that, I can only say that you’ve been warned.
Make that half the audience. Gotta start proofreading before I submit.
I see your dementia right here: Begotten
God disembowels himself with a straight razor. The spirit-like Mother Earth emerges, venturing into a bleak, barren landscape. Twitching and cowering, the Son Of Earth is set upon by faceless cannibals.
Watching this film is just as sick as the description implies. Fun for the whole family! 
I was coming in here to nominate Sante Sangre but I’d obviously blocked Island of Death from my memory (or Island of Perversion as it was titled on the video copy me and some friends rented many years ago). I find it hard to conceive of anything being more demented than that. What follows the goat-humping scene mentioned by Baldwin you’d rather not know. Seriously fucked up.
Every time I see one of these types of threads I wait and I wait and I wait… but it seems to take forever for anyone to mention Liquid Sky.
Or Head.
Or Green.
Or Mr. Mike’s Mondo Video.
Or The Groove Tube.
Or Twice Upon A Time.
Or… well, you get the idea. I can list demented/weird/strange films all day; I own quite a few and have seen even more. I have some kind of weird attraction to the weird which is apparently mutual; I seek them out, and they seem to find me on their own. I own all of these films on DVD, except Mr. Mike’s, because it was only released once, in 1985 on VHS, and Liquid Sky, because I haven’t gotten around to it yet.
My favorite of this (short) list is The Groove Tube, which even after umpty-ump watches still brings me to tears.
Sounds similar to Eraserhead, which btw, is just a parody of the Gospel story.
What really gets you is the cinematography. If you anyone ever rents Shadow of the Vampire some day, I recommend watching the “trailer”.
Re Mr Mike’s Mondo Video
I have a copy but haven’t been able to watch it in years. It’s on Beta, and all my Beta machines are broken.
One musical interlude is Sid Vicious singing My Way. But, claiming that the owner’s of the song wouldn’t let them use it, they show a few silent minutes of Sid singing.
Visiting the Mainland Temple Of The Perfect Wave, we hear the hymn “Were You There When They Crucified Jack Lord?”.
Re Lair Of The White Wyrm
What’s wrong with it?
Re Guinea Pig
The series is mentioned on Snopes. Apparently the first film Guinea Pig-Flower Of Flesh And Blood was realistic enough to convince many people it was a real snuff film. Then came Guinea Pig 2-The Making Of Guinea Pig 1.
Re Meet The Feebles
I saw this years ago. For months after you see this film, people will be able to make you lose your apetite (and possibly vomit) just by saying “Mmm, carrots. Are they yours, Harry?”
Re Bad Taste
Within the first 30 seconds of the film, a man’s head from the cheekbones up is completely destroyed by gunfire. “I’m a Derek, and Derek’s don’t run.”
Let’s not forget Liztomania.
Actually, let’s.
A Clockwork Orange, natch. It’s perfectly demented. 
Wellll…I suppose a dildo-wielding Princess of the Cheesy Giant Papier Mache White Worm and attendant tortured nuns is a bit tame next to gunfire-liquified crania you’ve nominated, but you must admit it qualifies as at least moderately demented…
Eek
Make that Dildo-Wielding Priestess. HUGE difference…
Actually, make that SPIKED Dildo-Wielding Priestess. VERY spiked.
Ugghh.
Which is spiked-the dildo or the Priestess?
I’ve seen it several times years ago but I don’t have a clear memory of that!