It would be hard to say. I read a lot.
I suspect that any list would be incomplete without at least one of Tom Clancy's right wing screeds. The more recent ones are far worse than his early work. Let's go with "The Tail of the Tiger."
It isn't so much his narrow world view that makes his recent work bad as much as his laziness in presenting it. His character development has become much worse over the years as well. Clancy is like Richard Burton (the actor) who was great sometimes, but usually just mailed in his performance. Both men seem to rely on their reputations more than their talent.
Another reason writers can publish truly dreadful work is that the book is one of a series that started very well and has a built up following. The science fiction/fantasy writer Anne McCaffery is also guilty of this. Pick any of her recent "Pern" books as example number two. Join them with most of Frank Herbert's "Dune" series for number three.
David Weber started out well with his Honour Harrington series, but he too has gone the way of Clancy. His recent books are all about "how much more can our poor hero suffer?"
All of the above started out well, and have produced good work before slipping past mediocrity into true dreadfulness. For a single work that stands alone as absolutely unredeemable I recommend (if that is the word) the work of JT Edson. While he has published his book under many titles the plot remains the same, and so many others have done the western so much better.
So: Tom Clancy "The Tail of the Tiger"
Anne McCaffery "Renegades of Pern"
David Weber "Field of Dishonour"
Frank Herbert "God Emperor of Dune"
JT Edson Take your pick. They're all the same.
Somehow I have omitted Bulwer-Lytton's "The Last Days of Pompeii," and Margaret Laurence's "Stone Angel." I excuse the lapse by pointing out that I had to read them as part of the curiculum in university. The others I read voluntarily.
