1st Edition AD&D - The original grey Forgotten Realms boxed set. The amount of fluff and hidden odds and ends about the world were incredible and inspired a lot of my initial days as a dungeon master.
2nd Edition AD&D - The Forgotten Realms deity books... still the best written fantasy religion breakdowns I have ever read. I wish any following edition took their setting's gods so seriously and provided them such detail. Even for my homebrew world, I used these books as a model for its mythology.
The Van Richten's Guide series for Ravenloft. Not only was it full of interesting stories and crunch, it breathed new life into old gothic tropes.
3rd Edition D&D - The Ravenloft Gazetteers by Sword & Sorcery Studios. These books were amazing, and you could tell the love for the setting that was poured into them. Written from a narrative standpoint with all the crunchy bits in the back, it made the Dread Domains feel like a living, breathing world. I was actually kind of pissed when WotC took the license back away from these guys.
4th Edition - I am sorry to say, for as willing as I was to give this system the old college try as a learning point for my kids, I found none of it inspirational. That's why I went to...
Pathfinder - I have to pick one? Frankly, the writing on this line is fantastic. Some of the smaller splat books are hit and miss; I'm not fond of writers who approach D&D material in a textbook format. But almost every supplement has some bit of lore, short story, or dossier that brings story ideas to mind.