There are two worlds that I can give valid comment on -
- Kingdoms of Kalamar
- Midnight
KoK: As far as production quality and material is concerned, it would be difficult to go past this. I have all the primary books for it as well as a few of the modules. The presentation of the material is close to as good as it gets. However, I honestly feel I'd be doing you a savage disservice by recommending it over Midnight.
The primary flaw of KoK is that it is too open. There are too many places doing too many things. This may seem interesting on the surface but it lessens the overall feel of a campaign. It relies on a vast amount of DM input to come up with a good enough story to engage players.
Midnight: Is a very different story and deserves your immediate attention. While the Kalamar presentation is excellent, it really comes down to substance for me. What can you pick up and read and become truly inspired by? For me, Midnight was head and shoulders above in this regard and it is this aspect that I look for in a campaign sourcebook. Just reading its pages gives you a hundred and one ideas for engaging situations to throw players in.
However, it mucks around with the basic rules and that won't be to everyones liking. Look at it and post back. I have a feeling it might be what you are looking for.
Best Regards
Herremann the Wise