The Basílica i Temple Expiatori de la Sagrada Família - Antoni Gaudí's most famous work. Construction had commenced in 1882 and Gaudí became involved in 1883, taking over the project and transforming it with his architectural and engineering style, combining Gothic and curvilinear Art Nouveau forms.
Gaudí devoted his last years to the project, and at the time of his death at age 73 in 1926, less than a quarter of the project was complete. The current projected completion date is 2026, the centenary of Gaudí's death. If you have not seen this basilica at least once in your life, you have not lived.