I would suggest looking through the posts in the Italy forum (travel) at http://www.flyertalk.com/
Some there say..
" Rome Luxury Hotels: Hotel de Russie, Rome Official Site, Five Star Hotel in Rome, Italy A Rocco Forte Hotel
Living in Rome I can tell you the St. Regis is not in the same class as the Waldorf Cavalieri or the Hassler. I've stayed at the Cavalier a few times and it was wonderful, albeit out of town. I like the Hassler location (top of the Spanish Steps) and of course it's the hotel that Tom Cruise and Katie Holmes stayed in when they came for their wedding at the castle at Lake Bracciano. The Hotel Russie, as suggested. is also wonderful, and a great location just off Piazza di Popolo. There are also many "grand" hotels on Via Veneto near the US Embassy.
Roma Airport limousine
For 35 Euros a driver met us outside customs, brought us to his nearby E Class Mercedes, and drove us to the hotel.
Another option is the Sheraton Roma. It is about 5 miles from downtown. It offer a courtesy bus to/from the City Center (Piazza Venezia) on a scheduled basis, with departures every 90 minutes. Also has an airport shuttle."
Phil & Liz