There is a resort called the Nai Harn in the far south of Phuket. It's a good four star option, and a Leading Hotels of the World member. It actually began its life a long time ago as Mandarin Oriental Phuket, before Amanpuri or any of the subsequently built resorts existed. It has a nice location.

But, in my opinion, the best location is the Northwestern part of Phuket, where resorts like Trisara and Anantara Layan are located.