Teatro di Marcello
A few years ago the city of Rome opened the renovated Capitoline Museums on its most famous hill. The “Grande Campidoglio” project featured a number of improvements, including the stunning underground Tabularium gallery that lies under the piazza and connects the palazzi via an ancient street. The Tabularium also provides the most stunning views of the Forum obtainable, in my opinion, since you basically can peer out from an opening in the sheer face of the Capitoline Hill over the sunken ruins. In addition, the roof of the Musei dei Conservatori is open as a restaurant and has some great views looking the other way — in this case towards the Janiculum Hill with the Theater of Marcellus, still my favorite example of ancient/medieval layering in the whole city, in the foreground.