If the question was, "Which campaign setting has the second-best fluff" then I'd be struggling very hard for an answer.
You didn't specify a D&D-based campaign setting, and this means that the running includes Glorantha, JRRT's Middle Earth, Fritz Leiber's Nehwon, the assorted worlds of Michael Moorcock, and Gary Gygax's Greyhawk - all campaign settings which possess an enormous amount of fluff some of which is very well-written. Glorantha and JRRT's Middle Earth in particular have decades of work that went into them; many of these worlds have their own custom-written systems (MERP, RQ2, Stormbringer etc.) as well as a variety of novels and other creative fiction.
In my view, Dragonlance, Forgotten Realms, Planescape, Ravenloft, Kara-Tur, Mystara etc. simply don't have what it takes to compete with those settings. There's nothing wrong with them but they're a little too derivative of the real masterworks... basically, where they're good they aren't original, and where they're original, they aren't really all that good.
Fortunately this wasn't the question. You asked which had the
best fluff, so there's a simple answer.
It was the first RPG world setting ever published (1975-76) from the days when D&D and RPG were vritually synonymous, and people are still working on it today. Visit the website at
http://www.tekumel.com
EPT > *