The Borghese Gallery is, hands down, the best museum in Rome. It's collection of Bernini sculptures and Caravaggio paintings are unparalleled, but it's the Pinacoteca (second floor) that really takes your breath away. In order to limit the CO2 that circulates through the room, a limited number of visitors are allowed in this section over the course of a day. Here is where Titian's "Sacred and Profane Love" hangs, a personal favorite, as well as a number of Lotto and Raphael masterpieces. Tickets must be reserved in advance, and a tour is definitely worth your money. Don't forget to check out the beautiful grounds surrounding the museum as well.