I think that Battlefield Earth has set a new standard for bad. But I also have to mention The Forbidden Zone. The only person in this you’re likely to have seen is Herve Villechaize. I once showed this movie at my annual Bad Film Festival, and my wife begged me to turn it off during the opening credits. Everyone else insisted we keep it on, but five minutes later, they also voted to kill it. Aside from a couple of posters on the SDMB, I’m the only person I know of who’s seen it all the way through.
Here’s a few… 
All’s Fair (1989)
American Gigalo (1980)
And You Thought Your Parents Were Weird (1991)
Bio-Dome (1995)
Bonfire of the Vanities (1990)
Book of Love (1990)
Boxing Helena (1993)
The Butcher’s Wife (1991)
Can I Do It Til I Need Glasses? (1977)
Cat Women of the Moon (1954)
Cheech and Chong’s The Corsican Brothers (1984)
Children of the Corn (1984)
Children Shouldn’t Play with Dead Things (1972)
Um… More to come! hehe… Ok, so maybe you’ll disagree with a few of those… 
I’ve said it before, and I’ll say it again: Highlander 2: The Quickening was the worst movie of all time, hands down. I have to admit, though, that I have not seen Battlefield Earth yet. And from what I’ve read and heard about it, I never will.
Then perhaps you haven’t seen “Several More Idiots in the Woods”, or “Blair Witch Two”. It actually worse! Hard to belive but true. All of the stupidity of the first one, with other, more traditional forms of movie stupidity thrown in just to be sure.
I am surprised that nobody ever mentioned the classic flop “THE CONQUEROR”, by the late Howard Hughes, ca 1955. This movie was so bad, that Hughes immediately withdrew it from distribution, after the critics panned it. I believe the cast included Dick Powell, John Wayne, and Myrna Loy. I happened to see it late one night on AMC-I don’t believe it is shown too often. Two main points:
-the plot is infantile (loosely based on the life of Jhengiz Khan)
-most of the cast later died of cancer-the flic was filmed in the nevada desert, around the time of the above-ground A-bomb tests. Speculation was that the stars inhaled a lot of radioactive fallout from the tests.
I repeat, it is one of the worst pieces of cinematic dreck ever produced. Mr. Hughes did well to keep it under wraps!
Ahem… I liked ‘Blair Witch Project’.
I liked the sequal, too, though not as much.
::backing away slowly::
By far the worst, most incomprehensible movie I have seen (almost 1/5th of) was Nixon, followed closely by a wonderful sleeping aid named The Bridges of Madison County.
Once again, I shudder in horror as I repeat: “Kiss Her Goodbye.” The movie so bad I had to once start an entire thread about it.
If they ever rerun it on AMC I will give you all fair warning.
Don’t miss the “sexy crazy chick hula-hoop scene . . .”
How about “Tales From the Past”? It’s a horror anthology movie that is incredibly badly done – bad writing, bad plotting, bad acting, and an almost nonexistent budget. They used some clips from Roger Corman flicks (like “The Raven”). But the thing that astonishes me is that they actually got John Carradine to do the narration! The man will do anything! I’m also amazed that I’ve never seen this film mentioned anywhere in lists of bad films.
We used to rent bad movies for fun based on the titles. It turns out you really can judge a movie by it’s box. We actually found a few gems though. Here’s the really bad ones:
Cyber Ninja - Not a ninja at all… very horrible… very gory… very dumb.
Fat Guy Goes Nutzoid - This fat guy, he goes nutzoid. Imagine a “learning disabled” adult wandering around on his own. No real plot. Not even funny-stupid. Very non-PC.
Cool as Ice - Vanilla Ice, 'nuff said.
Spice World - Spice Girls, 'nuff said.
Hot Potato - I think it is the sequel to Black Belt Jones (good movie). Either way it sucks.
The Avenging Disco Godfather - Rudy Ray Moore at his best (worst).
Christopher Columbus - The Discovery
Club Fed
The Cook, the Thief, His Wife, and Her Lover
Curly Sue
Cyborg
Cycle Vixens
The Dark Backward
Dead Heat
Desperate Hours (1990)
Dream a Little Dream
You couldn’t be more right - it’s a movie without any redeeming qualities whatsoever. Not only is it illogical, boring and with a plot consisting entirely of holes, its desperate aatempts at retroactive continuity means that I’ll forever be unable to watch the real Highlander without thinking of the lame explanations offered in this crap-fest.
S. Norman
Thank you spooje - I too liked “The Blair Witch Project”, but then again, I saw it in a small arthouse cinema at the Florida Film Festival, before all the hoopla from Sundance started. Definitely lost something in the transfer to big screen.
dantheman - You forgot D.O.A.. Not the 1950 Edmond O’Brien B&W classic, but the piece of dreck from 1988 with Dennis Quaid.
From Leonard Maltin’s Movie & Video Guide
“Noisy and needless remake of the film noir original…Badly overdirected by the Max Headroom creators; obvious red herrings abound.”
Bad movie, bad movie! Sit! Stay! Die!
Omigod, it was a year ago THIS WEEK, and yet the horror that is “Kiss Her Goodbye” is still emblazoned on my brain-pan. Click on this for some acid flashbacks:
http://boards.straightdope.com/sdmb/showthread.php?threadid=17819
The worst movie I ever saw in the theatre was Wing Commander. I’ve never walked out of a movie, but I came very close on this one. By the end of the movie, I was actually curled up in a fetal position crying “Make it stop! Please, make it stop!” I didn’t really expect much from a movie based on a video game and who’s target audience were people there to see the trailer to Phantom Menace, but I was in physical pain.
I saw Battlefield Earth this weekend. And while it was rather lame, it was not the worst movie of all time. It was not even close to the likes of The Blair Witch or Plan 9.
I kind of liked it, actually! Don’t ask me to explain - I guess maybe I never considered it to be 100% serious, even though that’s what it was striving for. I saw the 1950 version only recently, and I saw the '88 one a looong time ago. I’ll admit it’s not very good (and far, far worse than the 1950 classic), though.
Enid Is Sleeping
Everybody Wins
A Fine Mess
Firehouse (1987)
Folks!
Funny Bones
Ghosts Can’t Do It
Graveyard Shift (1990)
Henry and June
Hercules in New York
Hot to Trot
Hudson Hawk
Joe Versus the Volcano
I would like to second The Blair Witch Project…simply horrible.
A close second would be “Howard the Duck”, walked out on it.