I don't know if these are the worst books I've ever read - but they're the only ones that come to mind right now:
Daughters of Eve by Lois Duncan (for some reason I just hated that book, kind of a let down because I usually like Lois Duncan books)
and Wicked Lovely by Melissa Marr (I can't see why people seem to love this so much, it would be an okay story if it had a better ending - but because the ending sucked it made it hard to overlook the other bad things about the book - I talk about what exactly is bad about it in this blog after the youtube video)
Annnd um, Moon Children (at least I think that was the title) I don't remember who it was by, it was a point horror book I read years ago, it was kind of rubbish (some point horror books are actually quite good, that one just wasn't)
last one I can think of is The Silver Kiss... again, forgot who that was by - but I just didn't like it at all.
Don't know which one of those 4 would be the worst - but I didn't like any of them.