When I think of my "perfect" SF RPG setting, GDW's Third Imperium setting for Traveller almost always comes immediately to mind. There's a good reason for that. Despite the immense amount of detail that it eventually spawned, its core conception -- an interstellar empire so large that authority remains distant, thereby creating the necessity for individual action -- is eminently gameable. Indeed, it's probably the best way to create an interstellar sandbox setting ripe with opportunities for the player characters to seek their fortunes in whatever fashion they see fit.
And it all began modestly in a little 42-page supplement called The Spinward Marches and first published in 1979. (This isn't technically true, as details of the Third Imperium setting had appeared before the release of this supplement, but this was many Traveller players' first introduction to it -- no surprise given that it had over 40,000 copies over 14 print runs) This product describes the 16 subsectors of the Spinward Marches sector, consisting of over 400 worlds, more than half of which are Imperial worlds, the rest being divided amongst the Imperium's main rival, the Zhodani Consulate, various smaller states, and independent worlds.
Each subsector gets a two-page write-up, with one page providing a brief overview of the subsector and a list of its worlds and their game statistics, and another page presenting a map of the subsector and the express boat routes between them. It's an incredibly bare bones approach, leaving lots of room for individual referees to flesh out the Spinward Marches as they see fit, within the bounds of the basic data provided. Of course, Traveller's world descriptions are famously vague, with lots of leeway for individual interpretation of, for example, government types and other similar information. What The Spinward Marches is then is a largely blank canvas on which a referee can paint his own picture. Some elements of that picture have already been drawn in outline, but the specific shades and hues, as well as the fine details, are left entirely up to the referee to decide.
As a gaming product, The Spinward Marches has few peers in the realm of setting design. It's an incredibly open-ended, flexible supplement that can be used in a wide variety of ways. Moreover, it contains so many worlds that one could, quite literally, use it for many years without ever exhausting all of its possibilities. That was certainly true of my old Traveller campaigns, which generally took place over only three or four subsectors -- barely a quarter of its contents. If The Spinward Marches has a flaw, I'm not sure what it is, beyond that fact that several world names get repeated among its 439 planets. Certainly there's not a lot of detail here, but that's by design. Part of the fun for the Traveller referee has always been finding new and devious ways to interpret the alphanumeric world descriptions in ways to make the characters' lives "interesting" (in the Chinese curse sense) and this supplement removes a great amount of the tedium of having to randomly generate those descriptions oneself, since, in 1979, almost no one had a home computer to generate them automatically.
Though I'm no longer as obsessive about Traveller as I once was, I still retain much fondness for the game and products like The Spinward Marches are a big part of why. Its supplements were, by and large, truly optional and intended as aids to creativity rather than replacements for them. Likewise, the default setting of the game was remarkably broad and demanded that it be individualized by each referee in order to be fully usable. I can't help but love that, which probably explains why I'm reflexively skeptical of settings that provide lots of detail. The Spinward Marches proved you didn't need a lot of details to make good use of it -- in fact, it was better that way.