Here's Casa Mila, by Antoni Gaudi. It's a house, built between 1906 and 1910, and displays Gaudi's renowned and fantastic formal language. When I saw this I was struck by the undulating facade. It must have been a nightmare to draw and construct these geometries in the days before computer modelling. I love the way the building seamlessly wraps the corner.
Not satisfied with just a crazy stone facade, Gaudi designed every detail, down to the wild ironwork of the railings.
Selfie
Below is Casa Batlo, another house by Gaudi, built between 1904 and 1906.
I came back on a sunny day to get this last image. I love the way the sun and shadows play on the curvy stonework.