The name of this ancient civilization is<u><em> Harappan </em></u>and the best preserved and most extensively excavated archeological site is <u><em>Mohenjo-Daro</em></u>.
Explanation:
Mohenjo-Daro was one of the largest settlements in the Indus Valley. According to the scholar Dani A.H., this site was built around the year 2500 B.C., and had a remarkably civil engineering and urban planning. Under the decline of the Harappan civilization around 1900 B.C., Mohenjo-Daro was abandoned.