What is Rome's most ancient monument?

Rome’s most ancient monument is the Cloaca Maxima, an ancient sewage system that the Romans began building under the Etruscan kings in the 6th century B.C.E. It continues to serve Rome to this day. According to a historical study of it undertaken by John N.N. Hopkins of the University of Texas:

The Cloaca began as a monumental, open-air, fresh water canal…. [I]t served a vital role in changing the physical space of central Rome and came to signify the power of Romans who built it. (Hopkins 2007)

The Cloaca drained overflow water from the central Rome area. Before its construction, water from the river Tiber and from streams in the Roman hills inundated the valley every spring and flooded the site of the future Roman forum (ibid. p. 6). This regular flooding prevented the emergence of a coherent urban space. The sewage system enabled the integration of hill settlements into a single city. The Cloaca’s masonry includes archaic, mid-Republican, late Republican and Imperial strata.