Which RPG features the best writing?
This is a tough one. There can be many forms of "best" for "writing". Do you mean flavor text? Do you mean that the rules are clear with good examples? Do you mean the writing is evocative and makes you wants to play the game. Do you mean the writing is inspiring and makes you want to run the game? Do you mean the writing is lean and to the point? Do you mean the writing comes across as written by and for an adult? Do you mean the writing is not dry and boring? For the sake of this post I am going to assume all of these points are relevant.
White Wolf. Vampire and others.
I have always felt that White Wolf paid close attention to how they wrote. They seemed to approach their books as entertainment themselves. It wasn't just enough to give you rules and a setting so you could play a game. Some rpg books are nothing but tools for the gamers so they could make an interesting night of entertainment themselves. White Wolf instead seemed to want to have the writing itself entertain and be engaging. Sometimes they would get a little heavy with the prose, almost to the point of obscuring the game, but overall they were the best at presenting an rpg as something more than just rules.