La Sagrada Familia is one of Gaudi's most famous works in Barcelona. It's a giant temple that has been under construction since 1882 (that's not a typing error!) and is still under construction so be prepared to see a lot of work continuing when you visit. However this in itself is interesting, especially if you visit the museum inside the building. There you will see the stone masons at work carving stones to be used in the building.
La Sagrada Familia 's model