Without hesitation, the worst film I've ever seen is Ultraviolet.
It started off somewhat promising. An opening credit sequence done up in a comic book style. A cheesy action sequence that starts with a motorbike riding up the side of a skyscraper rendered with bad CGI.
After that first action sequence, the rest of the film turns into an absolutely forgettable borefest where nothing worthwhile happens, followed up with nothing continuing to happen, with maybe a few more scenes of nothing happening, before the film eventually begins to entertain the idea of MAYBE allowing something to happen.
By the time something DOES happen, the audience are already so fed up that not even the sight of a sword fight is enough to redeem the movie.
This isn't just any sword fight, however. It's a sword fight that takes place in the dark with both swords on fire. By default, that should automatically be one of the greatest action sequences ever committed to film.
Ultraviolet somehow managed to make even THAT look boring. An absolute crime to the world of cinema.
I already mentioned the comic book style credits at the start. That gave me hope that what I watching was merely a poor adaptation of a potentially decent comic book that I'd be more likely to enjoy.
Unfortunately, that was a false hope. The film IS the source material.
The closest thing there will ever be to a redeeming quality is the review my friend recorded years ago, where he tears into it with an impassioned rage.