Top Ten Best? Who cares? It's the bottom five that's interesting!
5. Speaker for the Dead - Riding off the popularity of Ender's Game is this much worse sequel. Ender takes his abilities from the first novel straight into the realm of magic healer as he makes people better and consequences magically vanish just by showing up. Capping that the novel is dependent on one of my least favorite plot holes: exceptionally clueless professionals. A biologist performing genetic engineering on a species and not understanding its life cycle isn't just stupid, it's insanely stupid.
4. Dreamsnake - All of the same problems as Speaker for the Dead but without the saving grace of being a sequel to a better book. A protagonist who fixes emotionally unbalanced people just by showing up? Check. Effortless success at everything by that protagonist as they wallow in self-pity? Check. Group of genetic engineers who couldn't find DNA if it was spat at them? Check.
3. Green Mars/Blue Mars - My other major science fiction pet peeve is bad economics and Robinson's Mars series is built on it. An economy built on extremely scarce resources requiring highly skilled professionals to maintain even basic services, a population distributed over the face of a planet, and high technology cannot run on people just giving things away. That doesn't stop Robinson from preaching quite a bit about how much better that is than modern day people. So it's annoying to begin with but then the story is built on the most unlikable cast of characters I've ever read about and three generations that cover 250 years.
2. They'd Rather Be Right - For the first Hugo winner they selected one of the best, for the second they selected one of the worst. The gist of the story was that Ayn Rand style objectivism when merged with the Scientology would turn someone into a superman. Coming at the height of the dianetics controverse in science fiction it's not surprising that this book got the attention it did but that doesn't make it any less preachy or philosophically creepy.
1. Hominids - It's like a stew of everything I hated from the above novels with the final topper that it is also very poorly written. Not just weak prose like They'd Rather Be Right which was at the average standards of science fiction magazines at the time, but painfully bad plotting, characterization, and structure. I've read juvenile novels that are written to a higher standard than this.