The correct answer is Harappa, excavated for 26 seasons, compared to Mohenjo-daro at 15 and Kalibangan at 10 when Gregory Possehl wrote in 1990: "Harappa has been the most frequently investigated of any of the ancient settlements of the Harappan cultural tradition. Since Sir Alexander Cunningham first excavated at the site in 1872-1873, there have been not fewer than 26 "seasons" of work at the site. This does not include the "digging" done by Deputy Superintendent of Police T.A. O'Connor in 1886. I have placed "seasons" in quotation marks since I am using this term to cover relatively short but important periods of work at sites as well as sustained campaigns lasting several weeks. In fact, if one considers only number of seasons and not extent of excavation, there appears to have been considerably more work at Harappa than at Mohenjodaro, the next site in this regard."
1. Harappa, view of Mound A-B. Courtesy of the Archaeological Survey of India, Punjab Photographic Volumes 509/86.
2. Harappa, area to the west of Naugaza's Tomb under excavation, Mound A.B. Courtesy of the Archaeological Survey of India, Punjab Photographic Volumes 510/86
3. Harappa, Mound B under excavation. Courtesy of the Archaeological Survey of India, Punjab Photographic Volumes 440/86
4. Poll Results, Dec 2015-Feb 2016 at Harappa.com
Gregory Possehl's entire article A Short History of Archaeological Discovery at Harappa describes the seasons of discovery at the site.
Images from Indus Age The Beginnings, Plates 2.37-2.39.