Given that most movies are bad, and that there are whole categories and sub-categories of badness - the sequel, the Madonna Movie, the Friday 13th Series, or Movies Starring John Travolta Before Pulp Fiction - it is almost impossible to choose a single film for worst movie of all time. But strangely, I do have a nomination and I believe it is actually the worst movie ever made. It is Boxing Helena. The director is David Lynch's daughter, and the film comes with the almost insane-making faults that the family connection might imply.
