.......
Since the whole d20 idea started an explosion of overall great work in the RPG world, I'd have to say that I think Scarred Lands and Rokugan top the "new" worlds in terms of caliber and cohesion.
The Scarred Lands (which I love to read, but will not DM for) has supplements that are very well written (not for spelling, but for scope and ideas). I really enjoyed the sarcastic preliminary notes of one of the NPC historians in the Ghelspad gazetteer. It seemed at odds with what has gone before it. The youngness of the Scarred Lands' history is interesting as well plot-provoking.
Its just a really cool retake on the standard fantasy world.
In the case of Rokugan, I see a great, detailed setting with an impeccable sense of unity in plot and theme, but I can't get around how hard it would be for me to DM and create new areas around that world. Granted I haven't given much thought to the matter, but I think this world will only be utilized in its true form by a few fans. Its too much of one thing.....
BTW, in the real world, I studied Chinese, was in China for the past six months, and generally think Chinese-based Asian fantasy is more fun to play in....so that might begin to explain my bias against Rokugan. I only hope Kara-Tur (or another broad-Chinese-based setting) will get its own sourcebook soon.....
Anyway, in the grand scheme of things my favorite world is of course my own campaign world, but just below that are Dark Sun and Greyhawk. Greyhawk, because its where it all started and that it has such a generic, yet rich history. I love Dark Sun because reading the original boxed when I was a kid literally took me into Athas. That's probably my single favorite gaming product up to date.
C.I.D.