La Sagrada Familia
La Sagrada Familia. The most beautiful and interesting piece of architecture I've ever seen. It's taking well over 100 years to build, and is now in its final stages. Gaudi became involved in 1883, a year after it first began construction. He is responsible for transforming it into a Gothic and Art Nouveau masterpiece. I don't know if the pictures below grasp the magical feeling of walking through the grand entrances for the first time.
Recommendation: Try to go on a sunny day, the stained glass windows reflect inside the building and it certainly adds to the experience.