I honestly recommend Las Letras apartment. We stayed here with my fiancé during our Madrid visit. Apartment is beautiful,comfortable,everything is in really good condition. Location is great,everywhere is within walking distance. Plaza Santa Ana is just around the corner with many bars and restaurants. In 5-10 min you can get to: Sol, all important museums, Retiro park and Mercado de San Miguel. If we ever going to come back to Madrid for sure we would like to stay at this apartment again.