I keep coming back to the Aquilae setting. This is a unique setting in that it really is a giant map with thousands of locations, dozens of kingdoms, and no rules. They sell a gazetteer that fills in the blanks, but I am happy with the map and doing whatever I want with the place.
This isn't for any rules system, and it goes as well with GURPS as it does DCC. Use 5E, A5E, ToV, LFG, or PF 1 or 2 if you want. Rolemaster or Palladium Fantasy? OSE or Index Card RPG? It works with anything.
It is just a map.
It could be more if you want, but it is just a map.
Every kingdom, city, dungeon, and fantastic location you make up for yourself.
Do a hex crawl or make it up yourself.
But this saves me a lot of work and gives me a canvas for my imagination.
Some others feel hopelessly stuck in the Pathfinder 1e era, though I have a precious few that held up pretty well, considering. Primeval Thule is one, but I feel this is weaker these days since Conan-like has been done to death. One drawback is this sameness to the kingdoms; referencing is knowing what is where is difficult. Compared to Aquilae, I would rather just make everything up myself than endlessly search through a book for something that may or may not be there. The newer versions of this setting feel a bit slick, as many do, and they lose something going into the 5E world from the more realistic PF 1e setting guides. I had fun with this using Savage Worlds Fantasy, though, so it is still on my shelves.
Another classic is the old Scarred Lands setting for D&D 3.0. This is a fun, primarily human-based setting with many cultural backgrounds and illustrations. This book puts a lot of modern setting guides to shame with its art and ease of use alone, and it is always a favorite.
But I have a lot of trouble using these with a few games. Thule is more pulp, so playing it with 5E feels wrong. Scarred Lands has this layer of realism and human-centric lands, so it feels better suited to a GURPS than a DCC. The flavor of these settings feels stuck in the era they were written in, which is better for some games and not for others.
Aquilae? This could be anything from a gritty, realistic GURPS setting to a gonzo 1970s DCC world. I can put my flavor on the map and do whatever I want. This could be Savage Worlds Fantasy. I look at this and see an A5E game. Old School Essentials works. Swords & Wizardry. I played Pathfinder 1e in a part of this world, and it worked well.
Whatever rules system I have, the map works well with it.
I can't say that about most settings.
Dungeon Crawl Classics reflects an extremely crazy world. GURPS is the ultimate in realism. The map does both equally well. Thule doesn't do that, Scarred Lands doesn't, and the classic D&D settings sure don't. That is a vast range of world support, and having places and towns to fill out myself saves me a lot of work. Even for DCC, the unique location names are thematic and could support many interesting gonzo DCC adventures. With GURPS, the maps turn into survival and exploration challenges.
I could even play this with Mutant Crawl Classics, though it would need more ruined areas. The deserts and badlands could serve as those, or this could be a world so far in the future many of the old cities and structures have rejoined nature, so these forests could be littered with the ancient superstructures of vine-covered skyscrapers and hidden ruins of places forgotten to time. Some cities could be hidden under the seas or in giant underground caverns.
Use your imagination!
What is in the above cities of Angrave, Trezona, and Prose on these 12-mile hexes? What is that fort? Is one an elven city? Who knows? Make it up yourself. Put dungeons in every hex. Make a cave map. Place some orc camps in the woods. Place Keep on the Borderlands around here somewhere. Use the GM layer of prepopulated adventure locations, or don't, and stick to the player maps and do your own thing. If you are playing DCC, place the adventures in hexes!
I could use random tables to create the towns, who lives there, and what their governments are like. I could make it up myself. I could make them friends or enemies.
The maps are the ultimate fantasy sandbox where you fill in whatever you want. It feels like the old 4E Nerrath setting, only expanded to an entire world. You can buy just the maps or get the setting book that fills in the details.