In my opinion the best chianina in touristic areas is available at Dal Toscano in via Germanico, near the Vatican. There are different restaurants with a similar name in other areas, better to check the street name before going there. I definitely suggest that one for meat's lovers and they have also other nice Tuscan preparations. For the meat I personally prefer their entrecôte more than fiorentina (big steak with bone). Reservation highly recommended few days in advance.
Via Germanico 58,, Rome, Lazio 00192, IT