Hyrule Field for The Legend of Zelda.
The Elder Scrolls series boast some of the largest Overworlds in gaming.
In
The Elder Scrolls III: Morrowind, the Vvardenfell island is a single continuous explorable location, dotted with countless entrances to smaller dungeon and indoors levels.
The Elder Scrolls IV: Oblivion is similar to Morrowind'', except that entire cities are also rendered as smaller sub-levels accessible from the overworld.