I would recommend Marta's apartment. We stayed here as a family of 4 for vacation before dropping my daughter off for her study abroad semester in Madrid. The apartment is very clean, located within 2min walk to the Metro, 10-15min walk to the main railway station and in a quiet pedestrian area of Madrid. The beds are comfortable, kitchen is well equipped, and the bathroom is clean.
Marta was extremely responsive via emails and txts. She provided great advice on various questions we had related to travel in the area, metros, dining, and areas to visit. She was a great help!
I would recommend this apartment to others who are looking for a comfortable place to stay while seeing and enjoying Madrid.