I have always rented from Avis in Europe. I generally check with one of the on-line travel websites to find what is cheapest and then go to the car rental website. Avis has generally been the cheapest for me. I just got back from Italy a couple of hours ago. We rented a car at FCO. It was great.

Be ware that at the gas stations near FCO, at least after dark, are self serve and take only cash and do not give change. You can't just fill up the tank. It was quite frustrating for us. We didn't have a lot of euros and not much change.

We stayed at the Hilton Garden Inn (much cheaper than the Hilton at the airport). It was quite convenient. The shuttle bus runs about every half an hour. It is somewhat confusing because there is a shuttle marked for the Hilton Garden that is not the shuttle bus. It's about a ten minute shuttle drive. Or 10 euro for a taxi for the impatient among us.