What is the worst experience you've had reading a book?

Not one book, but almost all of Asimov's Foundation series. The first one is one of my favorite sci-fi books of all time because I love seeing how each group has to use game theory to solve their own unique issue in order to survive and flourish as a society built on science and reason. While I admit that it's not always written well, I love the mindset that Asimov wanted to emphasize: violence should be the last resort for solving conflict between nations. When the factions outside of Foundation threaten them with war, they respond with soft power like economic pressure, religious sway, and focusing on making better advancements to science and engineering to defend themselves by being too valuable to destroy.

The fatal problem with the series arises in Book 2 though. Book 2 (Foundation & Empire) introduces the interesting concept of "what happens when a massive wrench is thrown into the meticulously calculated 1000-year plan?" Unfortunately, you can tell that at this point is when the concepts of the story become too smart for Asimov to handle, and he instead begins his trend of doubling and tripling down on deus ex machina characters with mind control powers for the rest of the series. All of the interesting methods of sociopolitical problem solving are thrown out the window to become sub-par adventure stories.

Books 4 and 5 (Foundation's Edge and Foundation & Earth) were written particularly poorly, and was probably the point where I should have cut my losses. The books follow not-Han-Solo adventure man, contain a sexist female sidekick that only serves to be a hot piece of ass for Asimov's self-insert character to have sex with, and then has an extremely uncomfortable "happy ending" where a traumatized child is left to be groomed by a robotic parental figure so that the robot can one day mind-wipe the child and insert it's own consciousness into their body. What's more is that they completely ditch the core premise of the 1000-year plan, and the ending undercuts any direction that the story could have gone from there.

The prequel books 6 and 7 (Prelude to Foundation and Forward the Foundation) aren't nearly as bad as 4 or 5, but they completely undermine the importance and intelligence of the character Hari Seldon from the first book. Instead of him being a great man and brilliant mathematician on his own, he's essentially led around by his nose by undercover robots that are the secret architects of everything just because Asimov wanted to tie-in elements from his books about robots.