Worst movie ever? That title belongs to Open Water - the SADDEST attempt at a "horror" movie I've ever seen. The most BORING plot, and longest anyone has ever had to look at the same scene for like 2 hours without very much changing in the background. I can only assume that it's sequel, Open Water 2, which I believe went straight to DVD, is just as bad.
Also, Army of Darkness was a TERRIBLE movie as well - never go see this movie. It's cheesy, lame, predictable at every turn, and it's not even funny.
On a far lesser scale, the Fantastic Four movies were kinda sub-par as well. The Fantastic Four were already pretty boring, possibly the least charismatic superhero group in the modern day, but the movie(s) made them look even dorkier, and possibly even more boring. Silver Surfer was cool, for like... half of the movie, before he got disabled/separated from his board.