This question required some serious consideration, mostly because I needed to decide if I had to name someone who has actually written a role-playing game or not. Since I like to make things difficult for myself, I decided to choose two writers to spotlight, one designer and one writer.
The designer I selected is Monte Cook, one of the designers of 3rd Edition Dungeons & Dragons and the science fantasy role-playing game Numenera. I know Cook might not be the most popular designer in the hobby. Hell, I know some people who downright hate his work, but I've always enjoyed it, especially his more recent stuff. Numenera is one of my favorite games currently on the market, and I still get excited when I hear he's got a new project coming out.
The writer I selected is Wolfgang Baur, the founder of Kobold Press and former editor of Dragon. The reason why I like Baur is that he has this ability to take something familiar and breathe new life into it, mostly by infusing it with the mythology of Eastern Europe. He made kobolds cool, which is a feat in itself. His work in Kobold Quarterly and the Midgard Campaign Setting really inspires me, and I feel it would be a disservice not to name him here because of that.