I nominate a movie nobody but my brother and myself seem to have ever heard of : Terrorvision, starring that kid who played Tommy Westphal.
Mitchell. A movie so bad that even MST3K couldn’t make it watchable.
Heartbeeps - Andy Kaufman & Bernadette Peters star as robots who fail in love and run away to start a family of their own.
Rabbit Test - Billy Crystal stars in Joan Rivers’ filmmaking debut as the world’s first pregnant man.
I think you mean, “starring Mary Woronov.”
Rabbit Test was Rivers’ first (and only) movie as a director. But she had written a movie before this: The Girl Most Likely to… which was surprisingly good.
I vote for this one. Although it’s funny enough that it might not qualify for worst of the worst. A movie with Bill Maher as a jungle guide? Adrienne Barbeau as a psychologist gone native, leading savage women cannibals?
Voyage of the Rock Aliens starring Pia Zadora and Craig Sheffer?
It’s going to be tough to find. 
Here’s a sample. Not sure what’s up with the pockets…?
Let’s Dance Tonight. There’s also a sea monster!
Robot Monster - the most hilariously awful movie I’ve ever seen.
Some movies aren’t so bad they’re good - they’re just bad. I’d put Santa Claus Conquers the Martians in this category.
The head of Nostradamus is stolen, and transplanted onto another body.
Wait, this is a thread about BAD bad movies, right? Party killers? How is Student Bodies listed here? I’ll agree about, “Mitchell”, except Joel and the bots DID make it watchable. It’s one of my favs. I’m notorious in my small circle of friends for my love of bad movies, so I’m rather disappointed I can’t contribute any new ones. Maybe because I avoid the truly bad ones, or forget them quickly.
I’ll back, “Eraserhead”, “The Room”, “Manos-the Hand of Fate” (THERE’S a movie Joel and the Bots couldn’t save - almost), and of course, “The Star Wars Holiday Special”. Wow, now there’s a film festival to slit wrists by.
Plenty of scope for taking the piss out of Battlefield Earth.
Are porn flicks allowed? If so, I recommend
***Captain Lust
Baby Face
The Erotic Adventures of Candy
Candy Goes to Hollywood
Blonde Fire
Slippery When Wet***
and
Tell Them Johnny Wadd Is Here.
Lizstomania. Franz Liszt battling Richard Wagner’s Nazi Frankenstein in a pipe organ fighter jet.
Jaw hanging bad movie spectacle.
Scouts Guide to the Zombie Apocalypse is pretty damn bad.
The Boy in the Plastic Bubble.
If Star Crash and Attack of the Giant Claw aren’t on the bill, I ain’t coming.
Then again, if Star Crash is in the mix, there will be a lot of the audience coming. Half for Caroline Munro, half for David “Mascara” Hasselhoff…
Man, I’m getting old. Used to be, I could come up with a laundry list of applicable bad movies, just off the top of my head. Now it’s all a bit cold and fuzzy. 
Well, in any case, I do have…Pulgasari! It’s basically a North Korean Godzilla knockoff propaganda film. Directed by a guy Kim Jong-Ill had kidnapped to make movies for the glorious DPRK. Some elements of the plot are suspiciously similar to Star Wars.
Riki-Oh: The Story of Ricky. Do not watch if lactose intolerant.