I thoroughly recommend Istanbul travellers to stay at one of Doret & Izel's apartments in Beyoglu. When I return to Istanbul, I will try to book there. It's incredibly convenient and accessible for all the touristic sites, but without being in the middle Sultan Ahmet, the old centre. They also provide an excellent and inexpensive car service to and from the airport, in addition to recommending first-rate places to eat and to visit.