Though not on South Beach I would highly recommend "Azul" which is downtown as one of MIA best its in the Mandarin Oriental Hotel in the "banking district" off Brickell Ave.

I also ate at "Nemos" on South Beach on Sunday which was very good and as the name suggests specialises in seafood. Its at the corner of Collins and Ist in the far end of the beach ( a bit away from the action) Just be careful you do not do what i did which was dropping the heavy copper plate-clad menu into your groin area.

mike