Dracula 3000. Though IMO the very last scene (ETA: removed it on the off chance someone wants to see it and doesn't want it spoiled for him/her) is so unintentionally hilarious.
The Blair Witch Project. I doubt that even MST3K could save this disaster.
Epic Movie. And I'm a fan of most satires.
X-Men 3. I found it almost totally disconnected from the first 2 movies in terms of character personality traits & inclinations, among other things.
As for the last, I'll go with
Matrix2 & 3--technically 1 movie, IMO, as 2 ended on a cliffhanger and got picked up a few months later by 3.
Honorable mentions, though...here's one I never saw, but I'll have to if I ever get the chance unless someone can save me from myself:
http://en.wikipedia.org/wiki/Star_wars_holiday_special
And I actually liked this one that was so bad it was good, but I expect many would need to bleach out their eyes after having seen it (btw, this multipage review is not only so true, I think it's the most hilarious review I've ever read!):
http://www.x-entertainment.com/worst.html
And after reading the page on the music video, if you feel perversely compelled to see if it's as bad as he says, you CAN find it on YT....