Basilica di San Giovanni in Laterano
As the official cathedral of Rome, it is the seat of the bishop of Rome, in other words, the pope. Oldest among the four papal basilicas, it claims the title of ecumenical mother church.
In 312, Constantine won his battle against Maxence and invaded Rome, stopping the Christian massacres. That's when he erected this basilica. Following several destructions (fire, war), the existing basilica was erected between the 17th and 18th centuries.
A curious fact about the basilica: In 897, the corpse of the Pope Formosus was exhumed and put on trial by Pope Stephen VI. The deceased pontiff was on a throne in his pope's vestments and was condemned for curious reasons; the corpse was stripped of its sacred vestments, deprived of three fingers on its right hand and thrown into the Tiber. The scandal caused by this strange trial ended in Stephen's imprisonment and his death by strangling during the same year.
The ceiling--from 1562--is famous as well as the wall of the right lateral door. The cloister from the 13th century was erected by Vassalettos and is among the world's most beautiful cloisters.