Most complete D20 Non-WotC Campaing Setting?

[Psion]

It depends what sort of support you are looking for.

KoK has more adventures, and a lot of background is tied up in the adventures. It also has a very nice atlas, but little in the way of regional supplements.

Scarred Lands is lighter on the adventures (though it has some). It has much more in the way of region supplements though, as well as two creature books, two magic books, and a "divinity" book.

KoK is more straight D&D, while Scarred Lands adds more to the meta-setting.

 

[DragonLancer]

I'd have to the back the Scarred Lands in this. Scarred Lands is one of the best D&D settings I've seen.

 

[Dr Bunsen Honeydew]

It's probably not what you're looking for, but in my opinion, Spycraft's setting, ShadowForce Archer is probably the best setting and rulesets that I've seen. I think that you're looking for a fantasy setting, but if you get the chance, take a glance through the Spycraft Espionage Handbook.