Everywhere in Corsica is nice.
Follow the weather forecasts to avoid the Mistral which can be every ten days
or so even in summer.
Sardinia is great especially the north and west coast from Porto Cervo in the
northeast anticlockwise round to Carloforte in the south. Cagliari is dirty and
not particularly friendly in my experience. But you'll have to pass through if you
circumnavigate Sardinia. Not much going on on the east coast.
I have cruised the north coast of Sicily but the south coast is more difficult
with silting harbours, although the Greek and Carthaginian ruins are impressive.
In my opinion Sicily is better explore by hired car.
When you've finished exploring leave the boat in Malta or Corfu.
Although a few people use Alghero in Sardinia now.
There's also a marina whose name I forget north of Crotone
but I doubt it's a place to winter.
The cheapest I've found in N.Sardinia is Castlesado (east of Porto Torres), last year peak season we we paying €17 a night for a 37 footer.
Very good shelter in the huge bay Porto Conte just around the corner north of Alghero.
David