Sure, the Pantheon is pretty impressive. But if you see determined Romans stream past around the back, paying little attention to the place, it's probably because they are on their way to Caffe' Sant'Eustachio for un caffe' vero. The constant queues are testament that this could be the best coffee in the world.
William Grimes back in 2002 wrote in The New York Times about the place:
When the need for a real espresso becomes overpowering, buy a ticket to Rome, tell the taxi driver to head straight for the Sant'Eustachio cafe. The espresso will be perfect. A little expensive, but surely worth the trouble.
For the pastry chefs among you, check out this informative video about making tiramisu, with Sant'Eustachio coffee of course.