Wow, lots of last airbender's. I didn't even think it was that bad considering some of the horror stories about other films I've heard about.
For me, it's Tentacolino or The Garbage Pail Kids. Lackluster animation, a terrible, insulting, brain-dead plot, and the worst soundtrack to any other film that I have heard in a while for Tentacolino. As for the Garbage Pail Kids... everything you've heard is true. It's so hard to watch. For me personally, I shut off Tentacolino in 40 minutes and The Garbage Pail Kids in 20 minutes. Tentacolino I shut off right after the song number with Pingo or Pongo or whatever, The Garbage Pail Kids after they unscrew the pipe and dump the sewage all over the main character kid. Neither of them I've been able to finish all of the way through to this very day.
Foodfight! is a close third, only I actually finished Foodfight! all of the way through. Not saying it wasn't bad, though. It was terrible. I just somehow came out the other end alright (probably because I watched it with my little brother and we heckled it the whole time).