Hard one indeed. At first, looking back several years ago (actually many years ago) I though "Artificial Intelligence: AI" was THE dumbest movie ever made. Couse, you know, the director (or whoever, i could care less about how ppl developing a film are named) died, so that Steven Spielberg took over and decided there should be some ALIENS (extraterrestials) somewhere along the way, so he fucked up the whole movie, which was already seriously bad. But enough of my hatred towards AI, lets call in the challenger: No County For Old Men. That movie tried to convince me of being the worst, taking the place of AI, but it was in a different way. It wasn't pointless alien featuring stupidity. It was pure pants-on-head retarded stupidity.
So it's a bit difficult deciding on who's the winrar on "The stupidest movie ever produced", but I would like to nominate them both. In my opinion, even though "No country for old men" comes close, it could never replace the classic place of "AI".
P.S. Whoever thought that "AI" is a great movie (or anything other than pure stupid for that manner) deserves to die of "The Gay" and be miserably forgotten and forsaken for all eternity.