Depends on how you define "flattest," but if you consider a place that is generally large and has little variance in height, it is Salar de Uyuni, Bolivia. In fact, it's so flat that it's used for satellite calibration.
Formed from prehistoric lakes, Salar de Uyuni is 4,086 sq mi. (10,582 sq km) large, larger than Delware and slightly smaller than Conneticut. The entire region is covered by a thick layer of solid salt.
During the seasons when the region is flooded with water, the reflections make it difficult to tell where the horizon is.