Places with tips and photos are the ones I've visited, others are suggestions from friends that I couldn't make it. The top 3 are highly recommended, must-see, don't-miss-it ones :)
Kılıççılar Sk. No:7, Nuruosmaniye, Fatih, İstanbul
Doner Restaurant · 287 tips and reviews
Mark G: A tiny place (not a restaurant just on-the-go serving), but serves some of the best doner in the city. If you're in the old city or visiting the Grand Bazaar, this is a must.
Historic and Protected Site · 144 tips and reviews
Mark G: This seems to be available only for corporate events and weddings. The location and building are amazing and the restaurant serves some unique tastes based on the traditional Turkish cuisine.