Well, technically it's tied for the best with Cantina del Gelato, but it turns out both establishments have the same supplier and serve the same ice cream. The owner at La Sorbettieria is very friendly and speaks excellent English, and part of the fun of going there is seeing what new flavors are available, as a couple of the flavors change daily. La Sorbettieria was our local haunt while we lived for three weeks in Florence, and on an academic trip a bit later, I brought a group of 20 college students there to taste it for themselves. Every single one of them proclaimed it the best gelato they'd found in Italy. Friendly service, generous servings, fair prices, and incredibly good ice cream—it's the total package. Try a waffle cone with salted caramel, caffe and buontalenti as your three scoop combo.
One note on location: Google Maps has it wrong, so if you're looking for it on your iPhone, note that it's along the wall on the Tasso square, right between two of the trattorias. Look for the awning directly across from one of the soccer courts and you've found it. Closed on Wednesdays.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.