Okay, just need to clarify: Worst movie I've ever seen, or movie I enjoyed the least?
On a technical level, Tommy Wiseau's infamous The Room is probably the worst movie I've ever seen, but I laughed myself silly at it and it turned into a thoroughly enjoyable experience.
That I enjoyed the least was probably Cirque Du Freak: The Vampire's Assistant, but that's my fault for watching the film version of my childhood favorite book series. The production values were fine, and most of the acting was fine (except for Chris Massoglia. Dear God, I've seen corpses exhibit more emotion than that kid), but they just butchered every aspect of the story. The only thing that resembled the book series were the names and, loosely, the plot. Just about everything else was changed. Now, maybe I'm too closed minded, but it seems to me that when you're adapting a series of popular books you should probably leave some things the fucking same!
I can forgive John C. Reilly for his part in it because, while he played his character the exact opposite of how he was written, he at least did a good job playing it the way he did.