While I haven't really done much with the Forgotten Realms #dnd setting in a while, it's 3E version of the campaign setting remains my favorite. Mostly because I am a really big fan of hyper-detailed settings, and the 3E version delivers that - covering all the regions of mainland Faerun in depth.

Though it also had some good setting changes from 2E that, while not as apocalyptic as the 2E "Time of Troubles" or the 4E "Spellplague" - certainly made the world more interesting. The Empire of Shade were a good villain for those who thought that the Zhentarim had grown stale, and the attempt by the Red Wizards of Thay to become a magical merchant empire provided some good story potential. For as long as it lasted, anyway...

Still, even if you play in later, post-Spellplague eras, the 3E and earlier material doesn't cease to be useful.