1E AD&D DMG. I don't know that anyone can match Gygax's style for making ideas pop into my head.
For second tier or sleeper contenders, I'm going with Aria: Worlds, Fate Core, or the original Eberron Campaign Setting.
Aria: Worlds tried something that I'd never considered, before: creating and playing a civilization through the ages with the focus on the narrative and roleplay possibilities, not strategic or war game. I never got a table of political science geeks together to actually play it, but a couple of us put it through some paces. If nothing else, it's probably the single best source book I've seen to try to flesh out "narrative settings" (i.e. cities, provinces, nations, tribes, or whatever cohesive group you want to describe), even if you're not going to "play" them as characters.
Fate Core really flipped a switch, for me, on how you could tie descriptive elements of the characters and setting into the mechanics. I picked up the book on a lark and haven't been able to get it out of my head, since. Unfortunately, I have one player who will do Fate for anything besides fantasy and another (young/new) player who just wants to get her arms around D&D before trying anything else. Fortunately, you can use the campaign generation/session 0 tools regardless of the system used for task resolution, so I'm making some slow progress and probably get some of the benefits of actually using Fate.
Eberron Campaign Setting probably isn't anything inherently remarkable, as a book. I have to include it, though, because I normally avoid published settings like the plague, especially for fantasy and doubly for D&D. I just prefer (strongly) to create my own setting. I also hate mixing magic as tech or having Magi-tech. But, I really love the Eberron setting. I can't put my finger on it, but it blew past two pretty solid "rules" that had held for over 20 years. Heck, after listening to the Manifest Zone (podcast) episode on the Daelkyr, I was ready to break another "rule" which is the Far Realms and D&D mix like chocolate and ketchup.