Oy my! I can think of several. Probably the worst would be "A Girl of the Limberlost". It's not a very well-known movie. It's more of an after school special. Trust me when I say it's absolutely TERRIBLE!
As for mainstream movies, I think the worst would be "Napoleon Dynamite". I know, I know, It's INTENTIONALLY stupid, but that was one of those movies where I watched it and thought to myself afterwards, "Wow, that was beyond stupid!"