Location, location, location. Air conditioning. A washing machine. And an 11-nozzle shower!
This apartment in Madrid was perfect for a short stay to explore the city, and the air conditioning was a great refuge from the scorching heat. It's just around the corner from the Atocha train station, which made for very easy access from the airport but was far enough away that we didn't have any noise from the train station. (Stop by the information desk at the airport and they'll give you a map with the train stop, should you need it.) The apartment is a few blocks from the Museo de Reina Sofia, and served as a great starting point for walking into the city and over to the Prado, Thyssen, and a slew of restaurants and plazas. There's also a subway stop a block away, where the subway line should be fully functional in another month; you currently have to transfer after a few stops due to construction on the line.
Overall, would definitely stay again. Great amenities and the location was perfect.