"If you can't say anything good about someone, sit right here by me." - Alice Roosevelt Longworth, on manners
"It's not whether you win or lose, it's whether I win or lose." - Peter David, on life
Right at the start of the thread the OP made the point that Neil Gaiman is the obvious choice as the guy who's got a big rep as a prose writer as well as comics.
That's certainly true.
Anyhow finally got round to reading American Gods this week, not having read any of his prose before. Its well written, efficient fantasy. But... I have to say... I'm startled at size of its critical reputation. It regularly makes top 25 fantasy lists... but I really don't see why. (For me, it's no where near as enjoyable as hundreds of other fantasies by likes of Guy Gavriel Kay, J V Jones, Phillip Pullman, Raymond E Feist, etc.)
Moore's Voice of the Fire stands out for me as by far the best book written by someone previously known mainly as a comics writer. Nothing else I've read comes close.
Ellis's Crooked Little Vein was good, but a bit lightweight; I expect something more substantial from him in the future, though. His first effort showed he has no problem writing in this new (for him) medium, so if he's motivated to do something a little more ambitious, I think he'll likely pull it off.
I had the misfortune to read Gaiman's comic book writing before anything else by him, and I've found it to be mediocre at best; thus I've lost interest in trying any of his novels.
Gaiman's main strength is the understated elegance of his prose, which makes the act of reading very enjoyable. But when it comes to plot, character development and tension, his comic-book work strikes me as far superior. American gods was something like Sandman redux, although more predictable. Ananai boys had me as worried about its protagonists as if they had been having a nice picnic in the Thousand Acres wood, which is unlikely to have been the desired effect. [I]Good omens was funny and irreverent, but also pretty predictable and lacking in the thrill department. All in all, my favorite Gaiman book is the least ambitious one, Neverwhere. It reads like a comic, but isn't apologetic about it. (And I say that as someone who worships Gaiman in his comic-book writer role. The sheer level of creativity and skill shown in Sandman, a level that kept rising as the series progressed, defies description).
I heard that his short stories are better than the novels, though, and I might try them someday.
edit: I see dupersuper beat me to the short stories comment!
It feels weird to me, how much I can't groove on Harlan Ellison's early comics writing much at all. So good in words, not so hot when coupled with pics and the Comics Code.
Wondering, now, who all broke into comics and prose fiction at roughly the same time. Dennis O'Neill, though more known as a comics writer was doing prose back in the day pretty much concurrent to his first comics work, including the novel that introduced the world to Richard Dragon. Marjorie Liu pursued her prose and comics career simultaneously (and wisely, through an agent for both), but she had a few novels out before any comics. Gaiman didn't put out a solo novel for ten years or so into his comics career (an adaptation of his script for Neverwhere, not an "original novel" per se, though obviously, he's adapting his own work), but he was publishing short fiction and nonfiction the whole time.
I have no idea if they're good, but I understand that Dan Abnett has written a lot of Warhammer 40 000 novels that had impressive print runs. (If they're anything like Guardians of the galaxy and Legion of the damned, then they're probably a good bet).
If I can damn him with incredibly faint praise, Abnett writes some of the best RPG tie-ins out there.
His original stuff is pretty decent, too. Embedded is a nice bit of satire that still works as traditional military SF and Triumpff is an entertaining, if lightweight, steampunk-y adventure.
Brad Meltzer of course. Although I'm not a big fan of his thrillers.