Sagrada Familia, a UNESCO World Heritage site and Antoni Gaudi’s magnum opus, is undoubtedly the most iconic structure in Barcelona (and the most popular, with nearly 3 million visitors per year). Construction has been ongoing for more than 135 years, and the surreal structure, with its rainbow-hued stained glass windows, is slated for completion in 2026. Even in its unfinished state, it remains an absolute must-see for every visitor to the Catalan capital.