There is actually no competition for this, the worst movie...of all time.. is "The Room" by Tommy Wiseau.
In an interesting twist however it is also the best cinema experience of all time.
To show how bad the movie is here is a clip of Tommy doing what Tommy does best, acting very badly.
Tommy put something like 6 million dollars of his own money into this movie because he thought he was making a master piece. He also thought he was a good actor, apparently he isn't a very good judge of...well....anything.
The room has become a cult comedy classic (not what tommy was aiming for) and still shows in cinemas world wide (including cinema Nova in melbourne). Seeing the room in a cinema is an awesome experience, because the whole idea is that you go there...have a few drinks and basically yell at tommy and make smart arse remarks at the screen for the whole movie. Its essentially seeing a shit movie with a couple of hundred people that are all there for the same reason. Great fun. The below quote sums it up nicely
”What makes viewing The Room attractive is the chance for audiences to gleefully ignore cinema etiquette while attending the late-night screenings. Patrons are encouraged to make witty interjections, heckle, holler and hurl projectiles (plastic spoons) at the screen. It’s the good-natured, interactive viewing experience that ensures, quite miraculously, that this deliriously incompetent piece of film-making winds up providing five-star entertainment.”