Watchmen. 

Birdemic. Even with Rifftrax it was hard to watch.

   
 

I have probably seen worse but this selection comes quick to my head

far worse

Edit: outsped :C

The Last Airbender then is my choice, they just killed it for me, I couldn't watch The Legend of Korra after seeing that movie...

Dear lord, I somehow forgot about these gems. XD

I was actually a tiny bit disappointed when I finally got around to seeing Plan 9, as while it was awful, I felt it's awfulness had been a tad overhyped, haha.

D&D though, good grief, how that film got made without somebody intervening is scary.

I found the awfulness of plan 9 kind of charming, but it was absolutely terrible, I didn't knew about the fame of the film when I watched, it was on tv late at night and I couldn't believe how terrible it was, that made me investigate and I discovered Edd Wood, then I saw the Tim Burton film about him and that one was fantastic.

What about Inspector Gadget? Is hard to make a worse movie than that.

For years before I ever saw Plan 9 everyone kept telling me it was the worst movie of all time, so as terrible as it is, my expectations of its badness had been built up too much. :p

And I only saw Inspector Gadget was I was like 12 years old. Back then I just thought it was kinda boring. If I saw it today I'd probably have a much better appreciation of its horribleness. ;)

Inspector Gadget is another childhood killing movie, the cartoon was great but the movie... bleah!

Another Childhood killing movie is Belle & Sebastien, besides the fact that I never saw the cartoon so it wasn't part of my childood in the first place, but hell was that movie awful, like 5 minutes into the movie I screamed "Oh, come on, what's gona happen next ? " and then I  said what I expected to happen to the dog at the end of the movie, I had to run when the torture ended when half of the women in the cinema thought I had already seen the movie and spoiled to them...

The Talented Mr. Ripley

I think it was called Bitch Slap. I won't deny I was interested because of the movie cover but movie was so unbelievably bad, I could not bear more than 15 minutes of it.