Mercurius
Legend
For whatever reason, among the early AD&D/BECMI-era settings, FR has stood the test of time, while Greyhawk, Mystara, and Dragonlance have all largely been shelved. A cynical view is that this only because WotC has thrown its weight behind the Realms, but then the question is, why? I think it has to do with qualities intrinsic to the setting: it just works better than the others for the default of D&D.
So, love it or hate it, the Realms is king for a reason.
So, love it or hate it, the Realms is king for a reason.