I stayed at the Marmara Guesthouse for almost a week and I have nothing but good things to say about it. If you're staying in Sultanahmet then this is the place to be.
Apart from the amazing location and comfortable rooms, i found each and every person concerned with the running of the place to be genuinely welcoming and kindhearted.
I would book a room here again on my next trip to Istanbul without a second thought.