Opinions will of course vary, but among my favorite third party settings include the Iron Kingdoms, Dark Legacy, Midnight, Freeport, and the semi-official Rokugan. Late in the 3.5 era we get Golarion, which was later updated to Pathfinder.Drowbane: Such as?
I really don't have anything whatsoever to do with 4e, but I really like that. That was the best innovation to 4e, IMO. I didn't care for the rules direction at all.I love PoLand. Not specifically the Nentir Vale or any of the published details. But the whole heroes-in-a-wilderness-of-horror theme? Heck yeah! I pretty much based my homebrew on it.
I'd ignore vast swaths of the rules as completely irrelevent to my game, handwave DCs, ACs and other stats as needed to generate opponents on the fly without actually having statblocks, and otherwise play fairly fast and loose.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.