There is not one. There are several itineraries - there are also "Rome Christian" with stops near the largest cathedrals, and "panoramic" with stops near many attractions (near the Colosseum, Forums, etc.), there is some route through Via Appia Antica. All these buses start and finish their tour on the square in front of the Termini station. There you can also buy tickets.

Bus is available, 110 Red Trambus Open. Working time 08.30-19.00. She walks every 15 minutes. The length of the route is 2 hours, the ticket price is 20 EUR.

Stops: Termini, Colosseo, Circo Massimo, Bocca della Verità, Piazza Venezia, San Pietro, Fontana di Trevi, Barberini, S.M. Maggiore, Termini