Bernini's Barcaccia fountain on Piazza di Spagna.
It would be difficult to choose because there are so many great places to see in the city.
I was in Rome for work and was planning to have dinner in the nearby suburb or at the hotel I was staying in. However, the hotel's restaurant was exclusively booked for an event. I also learned that they have a shuttle bus to go to the city center which includes a re-tour metro ticket. Things seemed to have fallen into its place for me that night. So I bought the shuttle ride on the way to the ancient city.
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