Airport (FCO) to Trevi Fountain at 10PM

Taxis flat rate from FCO is €48. Uber same or more.There is also the Leonardo Express train to Termini which is €14 per person so would cost more.

Flat rate taxi is best way at that time.

You asked for the best way and that is a private car service. After a long flight, some jet lag, not being familiar with the area, there is nothing better than having your private driver meet you inside the terminal and whisk you to your hotel. No fending off the "independent" cabs, waiting in the regular taxi line, or just having a "lost" feeling. It is not the cheapest way, but it is definitely the best. We can not say enough good things about Rome Cabs. It is a private car service (not a cab company) but only cost about 10 Euro more than a regular taxi. We always use them and have always been very pleased. Go to romecabs.com and see what you think. We always send an email and they answer us with a quote within 24 hours. You do have to pay cash (Euros) and the end of the journey but not in advance.

For 65 euro, Rome Cabs looks like a winner. At that time of the evening after what will have been a near 18 hour journey, its only the very slightest of splurges.

Knowing you have a dedicated car waiting for you to take you directly to your hotel will be a stress free end to a long travel day.

Most hotels can arrange a car for you. At that time you don't want any problems. Even if it costs a bit more.
Happy Travels!

The problem with having the hotel make arrangements is that you might wind up with a regular cab, a car service, or the desk clerk's brother in law. Happened to us once but there were four of us and only one brother in law so there wasn't a problem.

Another vote for RomeCabs. We have used them 3 times on different trips and love them. They are extremely reliable. The first time we used them was for a transfer from FCO to an apartment in the city center in the late afternoon. Unfortunately, our flight was 3-4 hours late and I was concerned that a driver wouldn't be there to meet us but they had monitored our flight and knew we'd be late. A very nice driver was there waiting for us right outside the terminal and he whisked us into the city center in a very comfortable air conditioned van. The other two times we used them were for early morning transfers from the city center to FCO for our departure back to the U.S. I was concerned that with such an early pick-up time (4 am) they wouldn't be reliable but both times the drivers picked us up on time and were extremely professional. RomeCabs is the way to go; priced reasonably, professional and reliable. Have a great trip!

We always use romashuttle.com and their private car service, which this past September cost 45€ from FCO to our hotel door. They meet you as you exit customs, take you to their car, and deliver you to your destination. Easy, easy...

Buon viaggio,

I guess that "fending off independent..." run can be translated as "saying no to licensed private drivers without a booked run". They shouldn't do it, and I refuse to use them for that very reason, but it's not an horde of Visigoths storming your personal space.
If you take a cab from the line, bear in mind that the fixed fare of 48 Euro is valid 24/7 and that it's an all-included fare. No need to tip if you don't want to.

Dariolab,
When I used the phrase "...independent cabs..." I was referring to the individuals who hang out near the exits claiming to be cabbies but really are not. No, there are not hoards of them, but we have encountered them in airports in both the U.S. and Europe. Have nothing against the legitimate taxis who populate the regular taxi line. Since you are from Italy, please know that my comment was not meant as a slam at the people or country of Italy. We love them both and go as often as we can. If you were offended, I offer my apologies.

TC no offense. I'm just trying to explain that in Rome those who approach you at the arrivals aren't illegal cabs. They are licensed private drivers (aka NCC in Italian) in a grey area.

By law they should work like romecabs: NCCs are supposed to take only the passengers they are waiting for, only those clients that booked their services in advance. If they were completely illegal, getting rid of them would be easier. I don't accept their services because they shouldn't work that way, but they are lightweights not dangerous outlaws.

You have nothing to apologize for.