Just one worst movie ever? I can't do it, I've got three that stand out. Movies that I stubbornly watched all of, hoping they would get better, only to reach the end and think, "I just wasted two hours of my life."
1) Disney's The Fox and The Hound - a contrived and utterly predictable plot, characters voiced by a bunch of has-been actors with no emotions left in them, the most poorly arranged and orchestrated soundtrack I've ever heard, and songs that don't deserve to ever be sung again (like "Elimination - Lack of Education"). This is Disney at the very lowest of their '80s slump.
2) There's Something About Mary - the movies tag line was, "Warning: The guys who did 'Dumb & Dumber' and 'Kingpin' bring you a love story." If that alone isn't warning enough, take heed: the humor is all stupid, obvious, sexual inuendo (mind you, I like sexual inuendo, in moderation, when it's clever); the soundtrack is a distracting, whining drone from some post-modern, pseudo-grunge band; the acting reeks - Matt Dillon sinks to new performance lows and the best performance is given by Brett Favre, who plays himself. Need I go on?
3) I can't remember the title, but it's some "B" movie (yes, I know we're not supposed to criticize "B" movies, but this was so awfull it falls way below the "so bad it's funny" standard). It's about a guy who repeatedly tries to commit suicide, finally succeeds, and ends up in either heaven or purgatory where the film finishes with a musical number called "It's Always Christmas in Heaven" that features Rockette-style dancing angels in Santa suits with the breasts cut out of them (the suits, not the angels). If ANYONE has seen this movie and can remember the title, please let me know - I feel it is important to correctly identify it so I can warn future generations of film-goers...