Gravity's Rainbow. Don't get me wrong, Pynchon isn't a bad author. I've read many of his other works, and enjoyed them. But GR is TERRIBLE. It literally makes no sense at all - it's an absurdist novel about people disappearing down toilets and V-2 rockets that strike on whoever has sex. PARTS of GR are good, but the whole thing, when strung together, is a complete mess. It's almost literary trolling - "hey, I'm going to write an insane book and see how many self-important critics decide it's brilliant because they can't understand it!"
I'm used to "weird" books. Hell, I've read Haruki Murakami and enjoyed some of his books, and let me tell you, Murakami books are WEIRD. But GR is just.... awful. I despise that book.
Now, to be fair, the actual writing in many places isn't bad. Pynchon is NOT a bad writer - Gravity's Rainbow is an example of atrocious storytelling and pretentious symbolism, but it isn't actually written poorly.
For the "worst-writing" in a book I've read, I'd have to go with Brian Herbert and Kevin J Anderson's "Dune" "novels". Those books are so bad, so absolutely atrocious, that I had to buy the rest just to see how they absolutely BUTCHERED dune with no shame at all. It was like watching a Car Wreck, you just can't turn away. To this day, I have never seen such a despicable abuse of a series - Brian Herbert and Anderson are like Vultures, picking apart the corpse of Dune for food. I.... I..... look, if you're a fan of dune, I advise you to read the "prequel" novels "written" by B.Herbert and K.Anderson, just to see an almost perfect example of crap writing. It's so bad. So very, very bad. Almost mindbogglingly bad.