We really like the Pera Palace if you like history and shabby chic.
It was the hotel that the Orient Express guests sayed in on their arrival at the end of the train line. It doesn't feel like it has changed much. It is however just over the Bosphorus from the main city sights. About a five or ten minute pretty walk over the bridge, but very easy to find and get to. Some nice local restaurants and bars in that area too.
On second thoughts just checked it out. It has gone ever so posh (expensive)! It was not refurbed when we stayed. Obviously! It is 25% cheaper if you book on bookings dot com though rather than direct.