Which car rental company is the most reliable?

I honestly can't name one rental company as the most reliable for all of Tuscany: service varies by branch and by season. To limit unpleasant surprises, I would book directly with a major brand like Hertz, Avis or Europcarat the counter of your arrival airport, rather than choosing only the cheapest offer from a middleman. Check the pickup and return conditions of the specific branch: they matter more than any general ranking.

Before paying, look at the amount of the deductible, the required deposit, the insurance exclusions (windows, tires, undercarriage) and whether your credit card actually covers these risks. At pickup, film the car and photograph every scratch, then have the damage noted on the contract; also keep a copy of the condition report upon return. A small car is often more practical on narrow roads and for parking in the Maremma. If you arrive in high season, book early: the choice may shrink and prices rise. Finally, avoid driving in historic centers marked as limited traffic zones: a rental car does not protect you from fines.