After being hounded by my sci-fi-inclined friends for years, I read Robert Heinlein's Stranger In A Strange Land. When done, I immediately went out and bought two hamsters and a cage so that something could rip that book apart and pee on it. If there was an editor within 50 miles of that thing, I'll eat my shorts. I'll eat yours too.
After I read that, I couldn't understand wtf everyone raved about, either.
But the absolute worst book evar is this:
I'd rather be shot in the head than ever read that ever again. Hell has a library where every book in it is that one.