I've just returned from a week's stay in Sardinia and I have to say that after covering most of the upper Northern part St Teresa Gallura is the best for me.
You immediately have a good feel about the place and it's surroundings. There's a lovely piazza there with all sorts of bar's cafe's and restaurant's. Just down the road within easy walking distance was our favourite eatery Restaurant Thomas especially if you like seafood. We stayed at Hotel Ancona a small personable hotel in town which was quite adequate and very clean and very reasonably priced. They also serve up a good breakfast but don't expect a cooked one, it is typically Sardinian and substantial.
There is a really lovely beach there with a fortress to the side. As you drive South along the coastal road there are numerous lovely bay's in amongst the rugged coast, you will also see some kite surfing going on in place's as the conditions are very good.
The sea is unbelievably clear and in place's it remains quite shallow until you are a fair way out.
We also did Bosa, Alghero, Stintino and Costa Smerelda which were also nice in their own way.
From Ste Teresa Gallura you can also get a ferry across to Corsica, the journey only takes an hour and doesn't cost much. It is a must especially if it's a nice sunny day.
What I saw of Sardinia is very unspoilt, hopefully it will stay that way.