I would say it might be the so called Left Behind Series. I’m a Christian and it was almost unreadable. Not because the story line was not entertaining, it just wasn’t told in a realistic fashion. If Clancy, King, Ludlum or almost anyone else had written the thing it could have been very good.
The Runelords, the first novel in a series of epic fantasies by David Farland, has a brilliant approach to world-building and magic, but the prose is just dreadful.
I personally didn’t think that Left Behind had a good story or good writing. Granted that the idea of the Rapture, while having no basis in the Bible, holds the potential for a good novel (though probably not a good series of novels), Left Behind had too many plot elements that were completly off-the-wall. One person controlling all the world’s banks, Zimbabwe joining the European Union, the United States ceding control to the U.N., and so forth.
The Lord of the Rings trilogy is probably the best example of this category.
Ender’s Game.
Sacrilege! 
Actually, I enjoyed Tolkien’s dry style, but I’ll admit it’s not to everyone’s taste. And if we’re going to talk about Tolkien, he doesn’t even make the top ten once you put in Great Russian literature: War and Peace, Crime and Punishment, to name two of the bigger offenders that I actually read (or tried to read, I did NOT finish WaP.)
Even in the realm of Fantasy - I’d say that Robert Jordan is worse, by far, than Tolkien. If only for the sense that the books are now (well, okay, as of the last one I’d read) just spinning out as perpetual motion machines.
I’d nominate the works of H.P. Lovecraft, even though I like them. I like them in spite of Lovecraft’s writing, however, not because of it. His ability to communicate the alien is unmatched in my experience, in spite of his stilted prose.
A large number of bestsellers are books that strike me as having great plots but when I sat down to try reading John Grisham, for example, his writing just turned me off completely.
But this is going to be a very subjective poll, I think.
Stephen R. Donaldson’s “Thomas Covenant” series made me wince. Likewise, Terry Brooks’ “Shannara” books. I loved some of the plot elements, but I just couldn’t ignore the wretched writing.
pinkfreud, I’d like to say, about the Thomas Covenant books, while I won’t suggest it’s great writing, it wasn’t the author’s writing that kept me from finishing more than the first book in that series - it was the character of Thomas Covenant, himself. Who was intended to be just as smarmy, annoying, and unlikable as he was. Which isn’t to say that the writing was bad, just not to my taste when focused on a character I was hoping would get killed.
Pretty much anything by A.E. Van Vogt.
Steven King seemed pretty awful for prose. It doesn’t seem to have damaged his sales though.
James Halperin is a futurist sci-fi novelist who has great plots and very intriguing concepts but couldn’t write a grocery list: characters are wafer thin, plots are left hanging, side tangents come for no reason, etc… His books include The Truth Machine (premise: how would it affect the world if a 100% reliable lie detector device was created?) and The First Immortal (primarily about cryonics, but deals with major societal changes over the next century).
The Invisible Circus by Jennifer Egan was an awesome story, but I found the writing to be at about a high school girl’s level.
Science fiction author Philip Jose Farmer has had some great ideas for books but his writing is so bad you feel he’s wasting them.
Tom Clancy is a dreadful writer in terms of his use of words, but he’s come up with lots of pretty good stories. “The Hunt for Red October” is a terrific story, and “The Cardinal of the Kremlin” and “Clear and Present Danger” are both very good as well. The man can think up a heck of a story.
But as a writer, he’s just brutal; he’s terribly under-edited, spends way too much time wanking over military kit, and generally uses words the way the Air Force uses area denial munitions. His personal opinions and politics - best described as “Not Really Bright Idealo-Fascist” - constantly get in the way of the narration. Any decent author, given any of the stories I cited, could have written a far better book.
Anne Rice’s omnisexual Eurotrash vampires would have been wonderful for a book or two or even three if written by somebody who wasn’t the Norma Desmond of popular fiction, but as it is she’s gone from “intriguing plots but pedantic and overwritten” to “same plot/different names from the last 10 books in an impenetrable jungle of purple prose”. I would love to see her plots taken on by a Jeffrey Eugenides or even a Stephen King (who can’t end a novel convincingly to save his life but at least knows how to keep your interest until the anticlimax). Rice’s pscho response to criticism on Amazon.com resulted in her offering to buy back her last book from disenchanted fans (which she reneged on when many sent it in) and defending her refusal to be edited.
Dune and its sequels. Unfathomably brilliant universe portrayed badly and at great length with .
We by Zamyatin - the first true totalitarian dystopia, dystopically described. Then again, maybe a big chunk of the shittiness lay with the translation.
Erle Stanley Gardener was a great writer of plots. He often built stories out of technical points of law, and I find it fascinating how unlike the Perry Mason television series, saving his client almost never involves letting the shining light of truth out for everyone to see. In the books, nobody knows the whole truth except Perry, and the method is not about exposing the truth as much as it is carefully controling the truth.
No lit majors here I see, otherwise surely someone would have mentioned Theodore Dreiser’s An American Tragedy and Sister Carrie.
Not that An American Tragedy was the best story, although it did make a good movie. But the writing was godawful.
I’ll be flamed for saying this by his legion of fans, but James Michener. I love some of the movies and miniseries made of his works (particularly Centennial and Hawaii), but the books themselves tend to be very dry with more geography than character development while the movies flesh out the people a bit. (The more interesting moments in Abner & Jerusha’s complex marriage in the movie Hawaii was courtesy of the screenwriters rather than the novelist.)
Right now I’m about 200 pages into Colleen McCullough’s First Man in Rome, and I have to say, it’s a pretty hard slog. The research is incredible and the plot seems pretty good, but the prose is abysmal- especially the dialogue, which seems to have been translated from Latin to English by an early, unreliable version of Babelfish. Frankly, I’m having more fun reading the glossary at the end; at least the entries are concise.
My perception may be damaged by the fact that the previous book I read was another extremely well-researched pseudo-historical novel: Quicksilver by Neal Stephenson. Say what you want about hisplotting, the prose is never dull.
Paladud, gotta go with you on this- Dune is fabulous, but you end up having to read it over and over just to make sense of it. Not in a good way.
I quite like the prequels by Herbert’s son and Kevin Anderson, mostly because they’re easier to read (if badly plotted), and I loved the whole Dune concept too much not to read them. I realise I am alone in this.