I don't think most RPG's have notably outstanding writing. To me good writing for a game should inspire one to want to play it just by reading a rulebook. That can be a real challenge as you're not writing a story, you're trying to tell other people how to play through a story! Bad writing can stand out but when it comes to rulebooks it's more about organization and layout than writing style when it comes to making a good book.
I suppose the intersection of these things can elevate or sink a book:
- The first Dungeon Master's Guide, the original AD&D DMG, has some organization problems but the writing is somehow really inspirational, at least for a lot of people. Style/art/eye of the beholder etc. notwithstanding a lot of people I see cite it as an inspiration.
- Shadowrun 5th edition is ripped in almost every review for it's terrible organization. Regardless of the quality of the writing the poor organization overshadows it to the point that for many it's a problem just getting through the game.
Of the games I think of when I think about the quality of writing:
- The old James Bond 007 RPG was pretty atmospheric at the time. I remember thinking it really sold the concept well.Flipping through it today it still stirs some interest in playing an agent who lives that high class life while traveling the world in defense of England.
- Shadowrun in general has done a pretty good job here over the years, at least through the first three editions. One example - integrating a comments section on each page or section long before we had common internet usage made it that much more interesting to read.
- MWP's Marvel Heroic is another winner here. I thought it did a really good job of integrating rules, examples, and setting into a cohesive whole.
- Right now I think Dungeon Crawl Classics does a really good job of this. There's a ton of flavor in the words in this book and it conveys a setting and a tone when the game really has no specific setting - that's quite a feat.
One thing I notice from my choices - I think it is easier to write better when you have a setting to build on and not just a rules system. References to people and places, references to historical or future events, equipment, even jokes are easier to integrate when you have a strong setting to work with in a book. Even D&D 5th edition makes specific references to D&D lore and is a better book for it.