Archbasilica of Saint John Lateran Italy

The Archbasilica is the most important of the papal basilicas. It even outranks the famous St Peter, as it is the seat of the Pope. Even though it's outside of the walled Vatican City, the terrain it's on is considered part of that country, not Rome. It is the oldest public church in the city of Rome, and the oldest basilica of the Western world. However, the façade is baroque, from 1735.

Metro San Giovanni is nearby. It's a bit far away than the usual touristic circuit, so it's usually less crowded than the average Roman landmark.