Golarion is the official setting of the PATHFINDER RPG. Now, I got a bit of stick for (a) including Golarion or (b) not including the 567,345 other non-official D&D settings. I'm sorry. I included Golarion because I remembered that it was originally created while Paizo was making official D&D materials, but I'm told I am misremembering. Mea culpa; but if it's presence bothers you, pretend you can't see it and slip in #11 in its place:
Spelljammer. Published by TSR in 1989. Magic sailing ships in space. Crystal spheres. Spelljammer attempted to "unify" all of the official settings and allow adventurers to sail from one to the next.
Released in 1995, Birthright allowed players to take on the roles of rulers and emphasized political leadership in the gameplay. Based around the concept of bloodlines, the game used multi-month turns to ortray national actions.
Back as far as 1980, Mystara - originally called The Known World - was introduced in module X1: The Isle of Dread. Over the years, a series of gazeteers expanded the world by adding new nations and other features. Mystara included the Savage Coast, the Hollow World, and set Blackmoor in its distant past.
One of the earliest true Adventure Paths, the Dragonlance series of adventures was also supported by novels in the 1980s. Authored by Margaret Weis and Tracy Hickman, the series of adventures depicted the War of the Lance on the world of Krynn. Dragonlance introduced the infamous kender race, and made death knights popular in the form of Lord Soth. Later, the setting expanded with dozens of novel lines, a new continent called Taladas, and then advanced the metaplot into later generations.
Inspired by the module by Tracy and Laura Hickman, Ravenloft was a gothic, horror setting located in a pocket dimension. The dimension was divided into domains, each with its own ruler - such as the vampire Strahd von Zarovich. Raveloft was launched as a campaign setting in 1990 with a boxed set called Realm of Terror.
Designed by Keith Eberron was the winner of the "setting search" competition in 2002 and was published in 2004. It featured pervasive low-level magic functioning as common technology (lighting, transport, etc.) in the form of magical lanterns and "lighting rails". It's a bit less high-magic and a bit more gritty than some other settings.
4) Dark Sun
Set on a fictional desert world called Athas, Dark Sun launched in 1991. Hihglighting psionics, thri-kreen, defiling magic, savage nomadic elves, and ferocious halflings, Dark Sun was popular with those who liked a dangerous, unforgiving setting and was a stark departure from the Forgotten Realms.
3) Forgotten Realms
The big daddy of D&D campaign settings, originally created by Ed Greenwood, the Forgotten Realms has scores of novel lines, video games, setting supplements, major NPCs such as Drizzt and Elminster, and a myriad of locations spanning almost every type you can imagine. It has been described by some as the "kitchen sink" of D&D settings, but there's no denying it's been the most successful. It was launched as a D&D setting in 1987, and has endured through several editions of D&D.
You know all those named spells in D&D? Mordenkainen, Dwamij, Tenser? They all hail from Greyhawk. Published as a 32-page setting in 1980, Greyhawk and it's world - Oerth - go back further than that, based on Gary Gygax's home campaign. Indeed, some of those names were characters in his game (and Drawij was Jim Ward backwards). It started as a castle and environs, with the insane achitect Zagyg at the bottom of the dungeon beneath Castle Greyhawk, but soon grew as cities and countries were added.
So, the most popular D&D setting of all time is set in a range of planes - both outer and inner - and centered on a city named Sigil. Published in 1994, and originally designed by Zeb Cook, Planescape also spawned what may be the most acclaimed of the D&D video games of the era, along with a CCG and various novels, plus six boxed sets and a handful of accessories and adventures. Planescape is some distance from the Lord of the Rings inspired public vision of D&D. Never as widely popular as the Forgotten Realms - or as widely produced, promoted, and supported - Planescape has long been the critics' favourite.