Fifty Shades of Grey.
Yes, I read it. I got bored one day during a tour and decided to read it.
It really was that bad. That's all I'll say about it.
Ayn Rand is up there. Granted, I didn't take it seriously when I read her stuff. It was so bad that I thought it was some strawman political satire, and I was thus actually laudably entertained.
That series is so bad it makes the Twilight books look like literary masterpieces.
And yes I read all of them. I have a really hard time not finishing books no matter how bad they are. The only exception to that is The Fountainhead. Or was it Atlas Shrugged? I can't remember, but I can't read more than 10 pages of Ayn Rand before I fall asleep or throw the book out of a window.
Other bad books I was made to read in school:
The Island - Gary Paulsen
Manon Lescault - Abbé Prévost (and in the original French, no less, which probably made it worse)
A Day No Pigs Would Die - Robert Newton Peck
The Golden Bowl - Henry James
Du Côté de Chez Swann - Marcel Proust (also in French, but it doesn't make any more sense in English)
Bread and Wine - Ignazio Silone
Tess of the D'Urbervilles - Thomas Hardy (not actually that awful a book, just awful for the main character)
The Order of Things - Michel Foucault (blessedly not in French, but it was still insufferable)
I think the moral of this story is that I wasn't cut out to be a French major. Thank god, I switched.