This article on the worst songs ever made me wonder what were some of the worst sf/f books people ever read. The only requirement is that it you had to actually finish the book. Otherwise, slam away.
For me, I would say that Battlefield Earth is the worst book I’ve ever finished. I though the first half was good. But then, at the halfway point, the main bad guy (Terl) gets killed off. The last half of the book details the efforts of Earth’s survivors to wrestle control of the planet from…shark bankers. Lame. Every page was a tough decision to persevere or give up. Unfortunately, I continued. After the read, I had no inkling to see the Travolta movie (although I did catch part of that train wreck on SciFi).