2e Dark Sun Campaign Setting Expanded and Revised (box set). I first became aware of Dark Sun from PotA’s appendix on adapting the adventure to other settings, and went looking for more. This was the first DS product I found, and while I would later delete it, it still inspired me to look for more.
2e Dark Sun Campaign Setting (box set) & 4e Dark Sun Campaign Setting. After reading the Expanded and Revised, I purchased these two products (2e PDF and 4e hardcover) and loved them both so much that I started building a 5e Dark Sun-inspired campaign right then and there.
I believe that the original 2e Dark Sun box set was the best D&D product ever made.
I could continue on with Dark Sun and similarly flavored sourcebooks (Dark Sun Monstrous Compendiums, the Ravaged Wasteland of Crifoth, B/X Mars, etc). However, that wouldn’t be fair to the other sourcebooks I draw inspiration from.
D&D Basic, Where Chaos Reigns & Twilight Calling. Technically adventure modules (set in Aelos and Mystara respectively), but I think they work better as a campaign resource.
Time traveling cyborgs, a domed city at the end of time, and several unique demiplanes? Heck yeah!
2e Time of Dragons (Taladas sourcebook). A sub-setting of Dragonlance. Great in its own right, and also a precursor of sorts to Eberron.
3.5 Eberron Campaign Setting. The first official setting I discovered on my own. I was so impressed by the world that next session I started telling my old DM about it, without knowing that he had the entire line of 3.5 Eberron products himself.
3.5 Secrets of Xen’Drik. This one was recommended to me by my old DM. I especially enjoyed the adventure sites chapter. The Sulatar are also the only interpretation of the drow that I actually like.
4e Eberron Campaign Guide. The two 3.5 products above made me a fan of Eberron, but this was the one that convinced me to actually run an Eberron campaign. It’s the best official Eberron sourcebook ever produced by WOTC in my opinion.
Monte Cooke Games’ Arcana of the Ancients. I consider this one to be a Numenera sourcebook.
When I’m running an Eberron campaign and the PCs enter the Mournland, this is the book I reach for. It’s got sci-fi items galore and bizarre monsters, all perfect for the Mournland.
Onyx Path’s Blood Sea: the Crimson Abyss. The cover art alone was so awesome, I went and looked into the rest of the Scarred Lands.
Pangolin Press’ City of Salt in Wounds. If ever I actually run a Scarred Lands campaign, I’m sticking this place in as the source of the Blood Sea.
Pathfinder’s Numeria, the Land of Fallen Stars & Iron Gods. What happens when you take Expedition to the Barrier Peaks and create both a campaign region and adventure path based on this classic module.
Seth Tomlinson’s Blades and Blasters. What if UFOs invaded your standard tolkienesque fantasy?
Dragon #315 & #359. I’m currently creating a wide variety of campaign ideas. These two issues have been helpful with the Isle of Dread campaign. I plan to use the Defilers of Ka as one of the enemy factions, and will be including an abandoned temple to Shaktari as a dungeon.