Daredevil has to be pretty close to the top. Not bad in a way to make it funny or anything, just an amazing festival of suck. (Dungeons & Dragons was a piece of art in comparison.) Ben Affleck is truly boring in any role, especially for such a well-known actor.
Spider-Man 3 was also pretty horrible, but at least it managed to do be slightly amusingly so. In animations, the recent Dragolance-disaster was really bad.
The Aviator was really bad too, even memorably so.
As for the classics, I would recommend
Glen or Glenda by Ed Wood. It's... Bad. Another candidate for worst ever could
Ilsa - Harem Keeper of the Oil Sheiks, or any other Ilsa-movie for that matter. Just generally unpleasant, without the touch of true absurdity or other redeeming aspects.
But if you want to know about some of the really bad stuff out there, check out
http://www.headinjurytheater.com/entertainment.htm