Spy Kids 3 was absolutely horrible. I'm not sure about the second one, but the third was horrendous. I mean, what was the point in 3D glasses? There's ONE 3D bit, and it's just a damn butterfly. It's like being promised as much chocolate as you want and only getting one bar of Galaxy.
I've heard that Dr Dolittle 3 was bad. I thought having the girl as the star would put a nice edge on it, but it was straight to DVD, and most of those are bad anyway.
This one may not count, but the film version of Harry Potter and the Goblet of Fire was the worst film in the series IMO. The first two were great, the third was OK, but the fourth was bad. It cut out many scenes, changed many parts and even cut some of the characters. Am I the only one who wanted to see Ludo Bagman as a character in the film?
There was one more...I'll edit this post later when I remember it.