I would say Frog God Games The Lost Lands, just for the amount of material. Their setting guide is also available to subscribe as a World Anvil world, which is very nice.
I got some items for that I think, Rappan Athuk? and Barakus?
How much stuff is on it?
Could you give me a list of the different books (or at least a starting point). I think I found the ideas intriguing from what I read originally, but no idea how much more there is out there or where to start.