The more I think about it, the more I'm convinced about which *major* motion pictures are the worst. To me, it really has to do with the amount put into the production, the expectations, and the quality of the work. Kind of like the ratio of the suckitude of a movie to the money and expectations that went into it. A flick that sucked badly, but had low expectations is not nearly as bad as a flick that sucked but had huge expectations.
In that light, without question, the worst major motion pictures are the first three Star Wars prequels, and the second and third Matrix movies.
I can't even begin to describe just how bad the writing and acting are in Star Wars 1-3. It embarrassingly bad. The plot lines are confusing and nonsensical, and the dialogue is horrendous. No acting could turn those scripts into something respectable, but it didn't matter, because the acting sucked. But given how high the expectations were for these flicks when they came out, the overall rank of suckitude had to be right at the top.
And then you have Matrix 2 and 3. Another example of movies that took a cool idea and made the plot lines confusing and incomprehensible, along with dialogue that makes me want to retch every time I hear it. This is seriously some of the worst dialogue I can recall. This wouldn't be so bad if the movies didn't really take themselves so seriously, but I never got that sense from these movies. Add to this the fact that the acting was led -- no really, led -- by Keanu Reeves (can you really call it acting, anyway?) and you have some of the worst movies ever in the face of their great expectations and the amount that went into them.
All that said, I still watch these whenever I see them on the tube. Why? I have no idea. Must be the special effects.