Though my fondness for Judge's Guild's Wilderlands has since eclipsed it, I still consider 1980's The World of Greyhawk Fantasy World Setting to be the best campaign setting product TSR ever produced. I can already hear the chorus of disagreement rising in the background, as fans of Planescape and Dark Sun, Birthright and Ravenloft, even the 1983 Greyhawk boxed set prepare to show me the error of my ways. They're welcome to try and make their case, but I'm not easily dissuaded on this point, as I'll explain.
When I say "best," I'm not speaking specifically about the Greyhawk setting itself. I certainly like Greyhawk a great deal, make no mistake. As one might expect, it's a perfectly naturalist setting -- a faux medieval Europe with magic and monsters added to it. It's the very definition of what hipsters nowadays call "vanilla fantasy." I readily concede that point. Greyhawk isn't very remarkable as a work of the imagination. Most of its setting elements are immediately recognizable to anyone with a passing knowledge of history and pulp fantasy archetypes. But that's to the good, since it makes it very easy for both players and referees to get into the setting without having to understand reams of backstory and minutiae. The World of Greyhawk was intended to be a backdrop for one's D&D adventures. Backdrop. It's there to provide a little context and depth; it wasn't intended to be the focus of one's campaigns.
Commendable though that is, The World of Greyhawk's real virtues are as a product. When I said it was "the best campaign setting product TSR ever produced," I meant that literally. This product consists of two large hex maps -- among the most beautiful ever made for any RPG product -- and a 32-page booklet that gives a brief overview of the setting, its nations, peoples, points of interest, and related matters. You'll find no uber-NPCs within, very little history, and no epic plotlines. With the exception of a few adventures that were tied to locations on its maps, The World of Greyhawk didn't even get further supplements.
The setting was a gigantic canvas onto which great swaths of bright colors had been applied, but which was noticeably lacking in detail. If the referee wanted such detail, he had to make it up for himself, which is exactly what I did do. When I tell people that, as a kid, I played in a Greyhawk campaign, what I mean is that I played in a campaign that used the map from The World of Greyhawk and the sparse details I found in the accompanying folio. The specifics of that campaign bear minimal resemblance to what Greyhawk would become later, with its great wars, grand plans, and powerful NPCs. Mordenkainen? Tenser? Robilar? Who were they? They never appeared in my games except as names associated with spells or magic items. I'm not even sure I was cognizant of their connection to Greyhawk at the time. My Greyhawk was home to Morgan Just, Sir James Calvert, Theinburger the Thief, and Evro, among many others. They were the most important people in the Flanaess and it was they who stood against its worst villains: Severinus the Lich-Lord, Ragrak Troll-Born, and Ashad Raghul and his Black Brotherhood. If you've never heard of them, that's no surprise, since I made them all up, placing them within Greyhawk and using them in all manner of adventures that forever changed the face of the setting. My setting.
They say that brevity is the soul of wit, but it's also the soul of a good campaign setting product in my opinion. The best ones are those that provide referees with the broad strokes and allow all the detail work to be done later, as needed, by the referee as his campaign evolves. The World of Greyhawk did that for me as a young person and I'm forever greatful for that. Much as I've loved many later campaign setting products for their brilliant ideas, none has ever compared with that Darlene map and 32-page folio when it comes to utility in play -- a brilliant product whose like I'd dearly love to see again.