Unfortunately, the car rental companies seem to attract employees who shouldn't be working with the public, we've run into those experiences as well. I suppose it comes from the high numbers of people they see every single day. So just be prepared, some Italians are great in the hospitality business, some just should not work there - but I believe that is true for any part of the world where you see people working in tourism and coming into contact with lots and lots of people on a daily basis. The reviews are based on bad customer service at pick up and return or for other services required during emergencies? Based on the reviews, take out the counter experiences since that will depend on who you find there on the day you arrive and you can't do much about it.
Haven gotten that out of the way, I would suggest start with the bigger car names such as Hertz, Maggiore, Avis and Europcar.
Also, are you arriving in Florence and want a car from the airport? I recommend that if you're not staying in the center, leaving to go elsewhere in the countryside.
If you want to first stay in Florence, you don't need a car so renting from the center would be useful, although driving out of the center is not always simple.... so even then, you can consider taking the tram back to the airport and getting a car rental from there in any case!