Best in Cala di Volpe, Costa Smeralda, Porto Cervo??
We, my wife and me, have been staying in Cala di Volpe, Hotel Petra Bianca, a few years ago. We had not planned to do so, but on our second day we rented a car. There is really not much in Cala di Volpe, besides some nice and simple beach restaurants and an excellent pastry shop up on the hill. Porto Cervo and Porto Rotondo are artificial tourist ghettos, with nothing remarkable outside the Italian mainstream. We finally made it to Olbia, which is a nice old town with a beautiful pedestrian mall in its center. I still do not recall any remarkable restaurants, only a small shop which sold aged Pecorino cheese.
The Slow Food guide lists now two places in Olbia: Barbagia and Da Gesuino, and a restaurant 25 km northwest of Olbia in Sant Antonio di Gallura: Da Agnese.
OTOH: we really enjoyed the dinners in our hotel, they were serving excellent Sardinian cuisine and wonderful, inexpensive wines. If you stay at the same hotel or at the hotel Cala di Volpe, which is a very famous five star hotel, I would stay there for most meals. They will be better than anything you can get within 25 miles.