Sant' Eustachio.

Reported to serve the best espresso anywhere, and I live about a ten-minute walk from this caffe (It’s in a small piazza right behind the Pantheon). Spendy, though. Three euro for this doppio espresso con panna. They have a rival about two blocks away where I can get a cappucino e cornetto (croissant) for about 1.5 euros. Va bene!