2e: Planescape boxed sets. Still the high water mark for creativity. World Builders Guidebook. Such a great tool.
3x: Cityscape. Great city supplement. Shackeled City. Set the design paradigm for the entirety of Paizo’s existence.
4e: Dungeon Masters Guide. A great DM’s guide for introducing and running games for newer DM.
5e: I’ve mostly skipped 5th edition but Lost Mines of Phandelver is the best intro adventure written for D&D ever. I’ve run it in multiple systems, used it to introduce kids to roleplaying games, and more. Just a great adventure tool.