any suggestions on what tour service for a one day privately guided tour would be?
If you truly want a private guide as opposed to a tour, I suggest contacting Francesca. I believe she is listed in Rick's Rome and Italy books. We had her as a local guide on our RS tour for Ancient Rome & the Borghese and she was incredible and very sweet.
If you're more interested in a general tour, do a search on the board. Angel Tours, Context Rome, Through Eternity, and others all have been discussed previously.