Assuming we're not counting our own homebrew settings, it clearly would be The Savage Frontier for me.
That little Forgotten Realms book can stand as a full campaign setting on its own, and the first version of the North from 1988 is a much more interesting place for adventures than all the later versions that timeline advanced all the best villains and threats out of it.
The map keeps impressing me more every time I look closely at it as the layout and the positioning of settlements and factions inspires plenty of situations.
Star Wars during the Rebellion Era is also a very cool setting.
I took the opportunity during my recent paternity leave to read the Grey Box and the early FR code Setting modules and Forgotten Realms Adventures hardcover...and I have been radicalized as an FR Originalist. Sticking to 1E FR and a few early 2E books (FR 7, FR9, FR11, FR13, the FR Atlas and FR Adventures) and ignoring the Time of Troubles and Oriental Adventures (cringe) leaves an awesome D&D Setting just as good as Greyhawk for me.Playable D&D - Grey Box era Forgotten Realms. Playable, full of hooks, plenty of room to make it your own.