Ptolus and Freeport. (Redhurst is my favorite mini-setting, suitable for being used with almost any other.) If I were in the market for another setting today, I'd grab DCC World from Goodman Games.
The only one not getting enough love is Kalamar. Or Erde.
Diaglo is right. The original GH boxed set is very different in feel than it does with all the later material created by others. In other words, the GH setting ran with only the boxed set material will be very different than a GH ran with all the material created for it.
Speaking of Gary, you may also want to give his Ygsburgh a look. Once he gets into the Castle Zagyg (Castle Greyhawk) there are going to be bunches and bunches of maps.
Iron Kingdom - they maybe a bit wonky when it comes to understanding the D20 rules (with the Mechanika and some of the classes and race issues) BUT man oh MAN the artwork and the great GREAT background writing just blows me away every time I pick up one of these books. It is by far my fav setting right now...
Conan OGL - good stuff, but then I love Conan stories in general. The books look pretty good and it can be pretty deadly and fun. Not playing anything in it right now but I still read and pick up most of the stuff coming out.
Freeport - I like the pirate city and the feel is very open to which way you take it. Looking forward to the True20 Freeport coming out soon.
Ptolus and The Iron Kingdoms.
I am running a Ptolus campaign right now and I would to try Iron Kingdoms at some point since I actually have some of the books.
Greyhawk, Westeros, Dawnforge, Midnight, and the DDC campaign world. I'd like to take a look at Wilderlands and Harn as I think they would suit my tastes.
Freeport - More of a island city than a setting but still a lot of fun. It can easily be tacked on to a bigger setting too if you want.
Wilderlands of High Fantasy - A huge and wild land. Plenty of room from the GM to come up with plots without the setting's metaplots getting in the way. Described by many as "old-school".
Kingdoms of Kalamar - I've never actually played in it but from reading a bit of the setting book it has historical feel to it. Definitely a lower magic level than the Forgotten Realms. There is plenty of support from Kenzer & Co. for the setting too.
Ptolus - Again a big city, but this monster has over 1000 pages of stuff written for it. The book also has information on the world of Praemal in it although you are free to ignore that if you want. At $120 it is a little pricey. On the other hand you do get your money's worth.