Largest charter base is Palma on Mallorca - and Mallorca as an island has got possibly largest number of bays to stay in - if you only have one week.
But Formentera is a 'not be missed' island and if that is where you wanted to go, Ibiza is the closest place to both land from a plane and charter from. Ibiza itself has possibly best night life. But no knowledge of charter companies operating there.
If you do enterr via Palma / Mallorca the only fleet I'd give negative comment on is (from 2007) would be Moorings. They then used Alboran Marina in Palma and boats looked pretty tired.
At the same marina there was 'Alboran Yacht Charters', a local firm but boats looked much new and sharper. Suggest maybe google
them for prices and compare with Sunsail and Nautilus?
We circled all the islands all summer in 2006 so lots of places mentioned on our blog if you go back that far. Sorry but all the older pictures have been removed to save space.
Send a private message if you want more info.