Oathbound is the setting that has most captured my imagination. The stack of adventures I have on my shelves, with little sticky notes and tabs, can finally find a home. As others have mentioned, the setting itself is detailed, original, and includes an "adventure path" edition in each supplement, but there is still room to add or subtract whatever you like. The world is made for adventurers, both in the literal and meta-sense. Most of the types of campaign you might like can be found in the Domains of the Forge.
Plus, it is the first setting that I have been able to sit and read cover-to-cover. The campaign book and supplements are well written and inspire me. The first novel is a good read and captures the feel of the setting too.
Eberron is a candidate as well. I like what is being done, but may have to see more before it gets up to the status of Oathbound. Plus, I have not been impressed by what I have seen of the adventures. I'll try the first novel and see what that does for me.