There is a big debate about this in Rome. The two competitors are Giolitti and San Crispino. San Crispino has fewer flavors but they are fancier. Giolitti is crammed with people all the time. I also like a chain called Blue Ice which is all around the city.
I don't know, but when I was in Rome with some friends we stumbled upon a gelateria with mostly savory (ei, not sweet) flavors. We tried tomato-basil, then garlic. YUCK!!!
honestly i'm pretty sure it's actually much better in florence. i have no idea why that is, but as far as gelato is concerned, that was my somewhat well tested (and tasted) experience. :)