Came in to mention Robin Cook, but Saintly Loser beat me to it. I hope he is/was a better physician than he is a writer.
I read one of David Baldacci’s books about 15 years ago, and I still regret it. Absolutely no characterization or establishment of scene.
Baldacci and Cook’s books were my introduction to the hard truth that a book doesn’t have to be well-written to be a bestseller.
Harry Harrison, maybe?
As far as the OP goes, almost every writer gets worse over time. It’s like they have only X number of good books in them, but when they run out they just keep on writing. Most if them start out (even most of the ones noted here) with a strong book which gets them published. and then they’re on the treadmill/gravy train.
They number of writers who produce many good books seems vanishingly small.
Really? You’d put Tolkien as worst with the likes of Ludlum, Rice, Baldacci, Rand, Clancy, and Dan Brown running around? Really?
Anyway, I assure you newly written sprawling novels with excessive wordage and rambling content are still being bought and published. Not that I agree Tolkien represents such in the first place…
ETA: Don’t believe me? Give Jean Auel’s The Land of Painted Caves a try.
L Ron Hubbard might be an option.
A friend loaned me a recent Robert Ludlum “thriller.” It was long, it was boring and, apparently, not even written by him. Never again.
I think for me, the worst writer doesn’t just have poor craft, vapid plot lines filled with cliches, wooden characters, and stilted dialogue. There should also be some just plain bad story telling: even a cliched plot can be great reading if it’s executed well. There should be a painful lack of real imagination. So therefore for me someone like J. K. Rowling does not really belong on the list, because there are some truly distinctive creations in her work, for all the constructions like, “‘open,’ he hissed…” Same with Stephen King, especially since it’s hard to argue that his work isn’t executed well.
My top nominations for this title therefore would be more the likes of:
Ayn Rand. Clearly an author whose work is kept alive only by fans of her political philosophy.
L. Ron Hubbard. Would be all but forgotten without help from the stupid cult-religion he founded.
E. L. James. I have no fucking idea why anyone buys his work. He represents the worst in fiction. Unreadable tripe.
I read somewhere of Child saying he doesn’t plot his books - I was thinking, I sure the hell hope not! It’s usually just Jack Reacher, Smelliest Man in America, Wandering Around Randomly Until It’s Time to Wrap the Book Up.
Of all the writers I read regularly Piers Anthony has to be the worst. His works not usually bad enough to stop reading but he definitely goes on the bottom of my list.
Lin Carter would have probably never been published if he hadn’t been the publisher. Strangely I like reading him even though he’s bad.
I enjoyed the first two series and their sequels by David Eddings, but the last series they wrote was absolutely horrible. I get the feeling the publisher threw a large briefcase full of cash at them and said “Write something, Please!”
??
Lin Carter wasn’t the publisher for the bulk of his work (or of any that I’m aware of). He was an editor at Ballantine, and was responsible for their “adult fantasy series” that brought back a lot of important fantasy writers into print (such as Clark Ashton Smith). But Carter had been successfully published long before he held that post.
I certainly wouldn’t count him as a “bad writer”. Og knows there are plenty worse than him seeing regular print. Nor was he, alas, ever really popular (although fantasy fans know him for Conan and Thongor, and for several other fantasy series now out-of-print.
Give her a break; she was born on Groundhog Day.
What is QX? Something from “Doc” Smith? I’ve only read a couple of his books.
I don’t think this is true. Stephen King became a better writer through practice. J.K. Rowling also became a better writer as the books went on. Practice really does make perfect 
I’m not talking about the quality of their prose. I’m talking about the quality of their ideas.
Can a person get better through practice at having good ideas (or at developing those ideas, or at culling the good ideas from the not-so-good ones)?
At any rate, I don’t think it’s inevitable that a long-time writer would get stale or run out of ideas, but it certainly does happen fairly often.
I would add Jonathan Kellerman to the list of authors who sell millions of books for no apparent reason. Absurd plots, poorly drawn characters, more useless stuffing than a Victorian couch.
Some authors write 20 books. He has written the same book 20 times, and the 20th version is as popular as the 1st.
Michael Crichton.
He has some interesting ideas, but apart from predictable plot and shallow characterisation, he includes an insane amount of technobabble filler which added nothing.
As more and more of his books got turned into movies, he increasingly started writing books with one eye on a movie adaption. And it showed.
That gets better with practice, too. What really happens is we like to build things up and then tear them down. We root for the underdog, but with only a few exceptions once the underdog wins we move on to a new underdog.
I’ll agree with this. Even when he started out, Crichton bothered me – he spent too much time in The Andromeda Strain trying to impress the reader with the wonder of Binary Code. Nonetheless, I liked his earlier works – Andromeda Strain and Terminal Man had interesting ideas, and I really liked Eaters of the Dead. But as time went by, Crichton seemed to be just half-baked. Congo was ludicrous, and the movie, from what I’ve heard, wasn’t all that great. I refuse to watch it.
There’s a reason Crichton never got a science fiction award of any sort (although he got two Poe awards for his mysteries).
Dan Brown and Nicholas Sparks.
If Tom Clancy cut his novels’ length in half they’d be great.
Okay - it’s not like I’ve read (or seen acted out) EVERY single Shakespearian play, but even my 10th grade English teacher (whom I didn’t really like) conceded that there “are many people who feel that Shakespeare didn’t do endings very well.” Ernest Hemingway? I dunno - I was never a big fan. The fact that he eventually committed suicide doesn’t help ameliorate the way I feel about him.