Roman Forum - seat of power and center of the ancient Republic of Rome (which is an earlier period to be distinguished from the latter era of Imperial Rome). This fixture in the heart od the city dominates even the modern buildings and squares - which are still the center of the national and municipal governments - and taken together, at least for me, could not be seen in one.
The Colosseum - iconic Roman monument, and a marvel of design and engineering, yet with a brutal and gruesome early history. Still an impreesive and imposing structure, I was surprised by its museum and what I learned about its occupancy after the Empire fell.
The Pantheon - once a monument to the gods since before the era of Christ, later claimed by the Church, and eventually purposed as a tomb ...and now a badass tourist attraction.
Ruins and reconstructions - with its own long and tumultuous municipal history there is no shortage of demolished, decrepit, reconstructed, repurposed, and restored (and often charging a fee!) structures all over Rome. Each of these has its own story, some whose spans dozens of centuries.
Craft and artifacts
Phenomenal artifacts from around Rome: some of the oldest evidence of writing found in Rome on an extremely well-preserved jar (left) from the Baths of Diocletian, and an ancient carved stone monument finely decorated with a Celtic-esque pattern (right) from the Theater and Crypta of Balbus.
And this, friends, is nary a scratch on the surface of what locals here and historians consider some of the real old stuff (some of the Renaissance statues excepted): the era of pre-Christian Rome.