The Moonshaes. I remember thinking how cool the Forgotten Realms would be when "Darkwalker on Moonshae" came out, with the little map and the quasi-pseudo-Celtic slant -- it was just nice and different.
After that, the Great Dale/Unapproachable East region.
Waterdeep is the most highly detailed RPG city I can think of. And most of it is pretty good. I have all the products, & have run entire campaigns without leaving Waterdeep.
I voted Cormyr (fun stuff happens in Cormyr when your player is a noble from the region) and Other.
Unapproachable East and anywhere in the Dalelands that doesn't feature Elminster. I'm looking forward to the Sea of Fallen Stars as well once the party reaches that point in the plot (and a high enough level).
Waterdeep, followed by Cormyr. Those two places and the stuff written about them (especially the City of Splendors box) most define the spirit of the Realms for me.
The Moonshae islands, definatley my favorite setting, Other then that I'll take the dalelands and the areas surounding. Or Cormyr, and Waterdeep is good too. Ah, anywhere I can kick the snot out of dragon cultists and steal their dracoliches horde is a good place for me