Florence Cathedral - This stunning cathedral church, or Duomo, was build by Arnolfo di Cambio and the famous dome was added by Filippo Brunelleschi, famous for his revolutionary design of a free standing dome. (this one being the largest of its time)
St Peter's Basilica, Rome - Mass in St Peter’s Basilica is in Italian and makes for an amazing experience for anyone. It follows in a Catholic tradition going back centuries. http://www.8thingstodo.com/st-peters-basilica-rome
UNESCO. The Cathedral of Saint Mary of the See, better known as Seville Cathedral, is a Roman Catholic cathedral in Seville (Andalusia, Spain). It is the largest Gothic cathedral and the third-largest church in the world.