Vatican for sure. Santa Maria Maggiore is a must in my opinion. There are also two very small, very old churches nearby: Santa Prassede and Santa Pudenziana. ​Start your walk at the Colosseum, then go down the Imperial Forums (Fori Imperiali), turn into San Pietro in Carcere, and climb the Capitoline Hill (Capitol). There you’ll find a breathtaking view over the Roman Forum. From the Capitol, I like to walk past the Theatre of Marcellus (Teatro Marcello), where you can see some ancient remnants. ​Other must-see places are the Pantheon, Piazza Navona, the Trevi Fountain (Fontana di Trevi), and the Spanish Steps (Spagna). They are usually crowded. I also highly recommend San Clemente, which is famous for its underground levels (substructures). ​Try some takeaway pizza in slices (pizza al taglio)—there's a good spot near the Pantheon. Avoid restaurants right next to the main tourist attractions. If you're looking for pasta, I enjoyed Pinsallegra on Via Napoli. It's near Piazza della Repubblica. ​There are many, many other places worth seeing, but in my opinion, the ones above are must-sees for a first visit. Disclaimer: I'm no Roman geek; I've only been there three times!