There are quite a few if I'm honest! When I was about 12 years old, I read Skellig, which amazingly has won awards?! It was awful. I second Joya's response; 50 Shades of Grey is so poorly written and the narrative is so unbelievable!
As with the best book ever, it's hard to pick just one. Dan Brown's The Da Vinci Code annoyed me a lot, because of the stupidity of the characters, who seem to know next to nothing about their chosen fields (Sophie is a code breaker but can't guess a riddle to which the answer is her own name!), the cliff hanger endings to every single chapter which just felt forced, and the conspiracy stuff which had been around for years being presented as if it was fact. For all that though it's not technically a bad book and I did manage to finish it.
Eragon, by Christopher Paolini was pretty terrible. It was extremely derivative and the fantasy world didn't feel consistent or interesting. It was written by a teenager and sounded that way. He either needed a better editor or to just wait a couple of years until he had matured a little more.
I think the worst ever is probably one of the stupid school readers we were given in primary school. There was this one about some kid who finds a horse standing in a creek and gives it a name. It was painfully tedious, but that was partly because we all had to sit around and take turns reading bits of it out and having everything patronisingly explained to us.
"He lay perfectly still in the grass, his shirt open over his sculpted, incandescent chest, his scintillating arms bare".
and I could go on...there's heaps of websites out there that has a whole list of horrible quotes from the book too.
By Tobin. I forgot the dreadful Post Birthday World which is so awful, long and idiotic that I can't bring myself to read We Need to Talk about Kevin, because I'll think of how awful her other book is....