There are a lot of RPG writers that I like, and a few that I love. Rather than pick a singular “favorite”, let me talk about the different ways that someone could earn that status.
First there’s what most people probably mean when they say “best RPG writer”: best designer. For me, that would probably be Robin D. Laws. I’ve loved every game he’s ever done (except maybe Pantheon, which I merely liked), and simply having his name on the cover will generally get me to buy a game book, without knowing anything more about the game. So far, I’ve never been disappointed by doing so. I both enjoy the games he designs and his writing style when presenting them.
(This is not a given. There are some other authors that design games I love, but the actual writing is an obstacle to that enjoyment, rather than supporting it. So just being a good game designer doesn’t automatically mean you’re good at writing it down. Most RPG designers are merely ok writers—their writing doesn’t hinder the design, but neither is it noteworthy.)
There are two other RPG writers that I could call “favorite writer” based on slightly different criteria.
John Snead I first noticed with what I believe was his first published RPG work, for Ars Magica back when it was published by White Wolf. Since then, I’ve been following his work, and while I won’t buy a John Snead work unless it’s in a genre or for a game I know I like (I don’t think there’s anything he could write that would make me interested in a Hunter: the Reckoning book), I’ve never been disappointed in the writing I have read. And if I see Snead’s name attached to a magic system, I know it’ll be good. I picked up Liber Ka because of his name on the cover, despite not owning Nephilim—I figured I could use it for some other game at some point. Even with Robin Laws, I wouldn’t buy a supplement for a game I’m not otherwise interested in.
Finally, there’s Epidiah Ravachol. I’ve generally loved his game designs but what sets him apart for me is his writing. I didn’t think I was that into sword & sorcery—I’d seen some movies, read some of the classic fiction, and tried out a couple of RPGs that targeted that genre, and they were fun, but that was it. Then I read Swords without Master. The writing is so evocative, and at the same time clearly conveys the rules, that it just sucked me in. Not only had he succeeded in capturing a genre that is apparently tricky to capture in RPG form, but he’d done it in such a way that I couldn’t put it down.
Another testament to Epidiah’s writing is his game fiction. I normally can’t stand game fiction—my eyes glaze over, and I end up skipping it. If there’s important game content in the game fiction (as opposed to presented in the rest of the writing), I’ll tend to miss it. Epidiah’s game fiction is literally the only game fiction I’ve read in an RPG where I wanted it to be longer. OK, sure, we’re talking micro-fiction in Dread, but I sometimes can’t even make it through a paragraph of game fiction before I start wondering if there’s anything important in it that isn’t elsewhere. Merely not turning me off in the span of a couple paragraphs is an accomplishment for game fiction; sucking the reader in in less than a paragraph is hard to do for anyone, game fiction or otherwise.