Well the real worst books never saw light. Or recently perhaps not. Self publishing is now possible at a reasonable cost and I'll bet some of them are pretty bad.
But those don't serve your purpose.
I'd vote for the entire Wheel of Time series by Jordan.
Or the Thieves World series starting with book 5 (I think, not positive just where it starts going bad).
Or the Gor series after book 8 or so.
Why for each of these?
Well Jordan writes pretty well, but goes nowhere. It is the neverending story, but not in a good way.
Thieves World was a cool idea. In a drunken after party at a Science Fiction convention the point came up that there was almost no production of short stories any more. It got pointed out that the fouindation work for a short story, creating the world and characters, was almost as much as for a novel, but the pay was far less. The outcome was the creation of Thieves World, a setting where different authors could use the same world and use, but not use up eachothers characters.
I was very good for a few collections. And part of the beauty was one could pick up book 3 and all the stories worked, perhaps not as well as if yuo had read books one and two. But worked. But then things changed. The stories were novellas or longer and it got to the point where one had to have read the previous book, and read it recently. All the virutes were gone.
Gor is an interesting series, fantasy in the Barsoom mold but with a bit of SM sex as a spice. Actually rather good. And book 5, Assassin of Gor even provides some really nice counter point to the physolophy presented. But starting in 8 or 9 the stories become weak and the SM becomes the mainstay. Spice is good, but few eat curry powder straight.
I also found "The Gripping Hand" by Niven and Pournelle to fall flat. It si a sequell to "The Mote in God's Eye" which was great. I don't remember much about it, other than that I quit partway through. (Unthinkable got a Niven and Pournelle book).
Honestly waht got me on this this line of thought is a book I never read. Misery buy Steven King. From what I know of it the story is abotu a rabid fan who is upset that an author killed off her favorite character and she holds the author captive and will use any means needed ot force him to revive the character.
King did not conjure up this idea out of thin air. It is actually a basic reallity of writing, all he changed is the motivation, usually it is more the carrot, that any book with the loved character sells (and publishers will even give a large advance).
So look to late books in series. They often fall well short of waht came before.
