If you can get it, and get it on an award... The Istambul Hilton works. It's near the big soccer stadium and cab service is plentiful. It's a short ride by cab to the major sites.
As a Gold got a nice upgrade to the executive floor. Great, friendly service and a nice restaurant/bar on the top floor. THey also have a good health club and the Turkish Baths downstairs.
A good choice.