It's surprising how much disagreement there is about what makes a movie bad. Seems as though everything that someone hates, someone else likes. I guess it supports the old saying that there's no accounting for taste.
I've been reading this thread, racking my brain to think of my all-time stinkers, and I really can't come up with a list. I've seen many bad movies, but usually because I wanted to -- I expected them to be bad, and was prepared for it, so they just lived up to expectations. I guess for me, a really bad movie is one that ought to be good, but fails. Perhaps the two classic examples for my generation are "Heaven's Gate" and "Ishtar".
Let's face it, what do you really expect from "Bikini Car Wash" or "Hercules in New York"? I agree totally with those who say that bad movies are often really funny -- campy bad -- but truly bad movies are BORING. I can tolerate watching Ahnold stagger through his two words of English in "Hercules in NY", or some poor slob trying to make a movie with $500 because even though the effects are cheesy, the acting dismal, and the story weak, they're just trying so damn hard that you gotta love 'em.
I know this'll create some controversy, but my candidate for the worst movie I've seen recently is "The Mexican". It felt like it was 5 hours long -- I was almost in tears, begging for it to end. A long time ago Gregory Peck made a movie called "MacArthur" that was excruciatingly boring. It's the only movie I've ever seen that succeeded in making WWII dull! To be honest, I despise(d) the latest "Star Wars" for the same reasons: it was a transparent marketing scheme devoid of originality, many (all?) of the 'stars' mailed in their performances -- was Liam Neeson on quaaludes the whole time? -- and it even succeeded in being racially offensive. Considering its budget, cast, and director, it was really BAD.
So, I guess I can't name my all-time worst movie -- frankly, I can't even begin to remember all the movies, good or bad, I've seen -- but I can say that for me, badness is a function of potential.