I'm not a big FR guy. Never read the books. Never played a D&D game in that setting. That said, I identify with it because of the Baldur's Gate Series, and it has a lot of merits and many good books.
Grayhawk never made much of an impact to me. I've had mediocre experiences with Dragonlance, though I'm sure it's a fine setting. Don't know anything about the other two.
If I had to use a published setting, I'd probably use Kingdoms of Kalamar. If I had to use one of *these* settings, I'd use FR.
Specifically, what I like about FR is the cultural diversity and the great campaign setting book. I also like that it embraces the tropes of D&D (i.e. magic is king at higher levels and the world economy is based on it). I don't like the amount of canon one has to wade through or the recent gutting of the setting, but those are both easily ignored by a capable DM.