6. Basilica de la Sagrada Familia in Barcelona, Spain
The Sagrada Familia is the one cathedral you must see in the entire world. This Basilica was designed by the renowned architect Antoni Gaudi and is a masterpiece inside and out.
Gaudi wanted a church to make a statement on the skyline. The Sagrada Familia is certainly one of the tallest buildings in the entire world.
It is still under construction and will be finished in 2026. However, it has been in progress for more than 130 years. You should visit it no matter whether it is complete or not.