In theory, there isn't one. In practice, it's the Realms. Most official products are Realms-focused. Tales from the Yawning Portal - the Yawning Portal is set in the Realms. Tomb of Annihilation - set in Chult, in the Realms. All the adventures other than Curse of Strahd are Forgotten Realms based, though at least some of them have advice on moving them somewhere else.
WotC has even doubled down on the Forgotten Realms this year with two Waterdeep-based adventures. On the other hand though, they've also published Wayfinder's Guide to Eberron (DM's Guild) and the recent Ravnica book for playing in the Magic: the Gathering setting.