The best gelato I've ever had was at Santa Trinita. It's across the bridge very close to the Ponte Vecchio
Perfect creamy texture and great well pronounced flavors without being overly sweet. The fruit flavors were outstanding as well as the more traditional chocolate and cream based gelato. My favorite fruit flavors were mandarino and lemon. The sign says all fruit flavors were made with at least 50% fruit. Tart, sweet, refreshing and delicious.
My favorite flavors on the non fruit side were stratichella, chocolate and the peanut/chocolate. I absolutely loved the peanut and chocolate flavor. Reminded me of a great candy bar - pure flavors without the sweetness. You did not need a glass of water after any of the gelato flavors. The pistacchio was very good but I'd actually say that Bulgarini in Altadena was actually better.
I also tried Carraia down the block a few times. The lines at Carraia were actually longer but I preferred Santa Trinita. The ladies on the bridge at one of the jewelry store on the Ponte Vecchio said Santa Trinita is one of the best in all of Florence. While my wife was buying some jewelry I passed by 6 gelato stores to get to Santa Trinita and it was well worth the extra steps.