Hands down - worst movie ever (new to me) was "Crash" with James Spader as a weird sex and automobile crash film. It was about as much fun as checking out someone's raw scar from a recent operation. I actually felt queasy during the film and watched some of it in Fast Forward. Bleech!
Number 2 would be the "Blair Witch Project".
Three could be any number of recent "poor" films including: "Next Friday", "Deuce Bigalo Male Gigalo", "The Beach", "Ghost Dog", "What Planet are You From?" or "Stigmata".