My favorite church in Rome is in the heart of the bohemian neighborhood of Trastevere. It's not the most impressive nor the most fraught with artistic treasures, but for me it's simple elegance makes it the most beautiful. This is the church which lends its name to the whole neighborhood and the gilded ceilings remind me of an Orthodox church. You can literally feel the devotion here.
Many people that visit Rome choose to toss a coin in the Trevi Fountain or put their hand in the Mouth of Truth.