My mother and myself stayed at Sergio's apartment in Madrid and we loved it!
It's located in a very nice neighbourhood where you will find all the shops and amenities you need. There are a couple of metro stations within a few minutes walk, but it was also very easy to use the bus or simply walk into the centre of town.
The apartment itself is tastefully decorated, has all the modern appliances and good wi-fi. The building and neighbours are super quiet so you can definitely get a good night's sleep.
Thanks Sergio!