I usually go through AutoEurope.com (a consolidator) or their affiliate www.kemwel.com (whichever is cheaper on the dates I need, but both work the same way, since they are part of the same corporate group).
When you use the above consolidators, their best deals are invariably with Europcar or Hertz, especially when you select the zero deductible insurance option (which I always get for piece of mind, since it's only a handful of extra euro a day).
Europcar (a French multinational) is the largest operator with the most offices in Italy, but Hertz is a close second, therefore I go with either one, whichever is cheaper on the dates I need. Avis-Budget is usually more expensive, at least for the dates I needed, for whatever reason. I've rented from them only once.
I've never used Rhodium, which is a new brand of Goldcar, a Spanish rental company (GoldCar has an awful reputation). Besides I feel more comfortable with the aforementioned companies, that have been around forever. I remember Europcar, Avis, Hertz being present in Florence since I was a kid (and I'm no spring chicken).