A few favorites:
Plaza Santa Ana.
Oldest restaurant in the world. Restaurante Sobrino de Botin.
The beautiful architecture.
The hustle and bustle of the city.
Parque del Buen Retiro.
Tapas and more tapas. Paella, jamon iberica, potatas bravas, octopus and croquettes, to name a few. Recommendation: Taberna Alhambra.
And finally, although not photographed, amazing museums such as Museo del Prada and Reina Sofia. Bosch's El Jardin de las Delicias may have changed my life.