The Chigi Chapel, designed by Raphael with statues by Bernini, Adoration of the Child by Pinturicchio, and 2 more Caravaggio masterworks at either side of the altar, Conversion of St Paul ,and Crucifixion of S. Peter. A must, in my opinion, both churches, for anyone interested in Italian art, and of course free too. I would agree with lodington! you know where Caravaggios painting used to be? In the St Peters basilica! It was on display for just one day but the church deemed it to "exotic" due to its nudity and then moved it to Piazza del Popolo... So difficult to choose, but for me, a number of them are in The Capitoline Museums, in particular the statue of Emperor Marcus Aurelius (copy outside, original inside), the bronze she-wolf who nursed Romulus and Remus, the marble Dying Gaul and Carravaggio's depiction of the young St John the Baptist. I love your 3's questions. With Rome's embarrassment of riches, this is nearly impossible to answer. I'm sure I would give an entirely different answer each time I was asked, but right now...(and excluding buildings from the category)...Bernini's "David"; Michelangelo's "Moses" ; Carvaggio's lifecycle of St. Matthew. If buildings are included, you'd have to choose the Pantheon and Bramante's Tempietto, and don't get me started on ceiling frescoes.