Sagrada Familia, Barcelona

The Sagrada Familia outperforms #8 Big Ben in visitor volume, drawing over 4.5 million people each year to become Spain's most visited monument. Construction began in 1882, and completion is anticipated by 2026—144 years after the first stone was laid—making its timeline 40% longer than the average UNESCO project. Antoni Gaudí's basilica features 18 spindle-shaped towers, the tallest of which will rise 172.5 meters upon completion. Its unique organic forms and intricate facades, which combine Gothic and Art Nouveau styles, evade easy comparison; however, the sheer number of construction years surpasses every other building on this list, underscoring its singular architectural ambition.