Giancolo. Hands down, best view. You can hike up the hill (it is just a hop, skip and a jump from the Vatican) and see the entire city and all the way to the Alps. We were lucky that our wandering around/ getting lost allowed us to make it to the top of the lookout right at sunset. The golden city looked even more amazing from such a height. It was really cool to go to Giancolo at the end of our stay in Rome so that we could spot all of the sights that we had previously toured on eye level.
To all tourists/ travelers out there, take the time to go up Giancolo. Enjoy the view, enjoy the little park and monuments. Find out where the Zamboni lives :)