If I were given the second time to visit Madrid I would without a second of doubt go for Bibiana & Gigi's! -
It is right in the heart of the city with Puerta del Sol and Plaza Mayor in minutes of walk. It is beautiful, beautiful atmospheric flat of creative individuals (not to mention the fact that it is neat and clean and well-maintained).
Most important thing, however, is the hospitality and openness of hosts. Thank you for having us!)