That would probably be St. Peter's Basilica in Rome, where Il Papa says the Mass.
There are many of them around the world. Most important are the ones in Ireland, like St. Patrick's Cathedral in Armagh, the seat of the Catholic Church in Ireland.
If population membership is the sign of importance then it is the Roman Catholic Church.
martin Luther
Catholic church
The single most important religious order of the Catholic Reformation was the Jesuits.
Westminster Cathedral.
Roman Catholic AnswerThe most important of the sacred signs in the Church are the sacraments.
They either left the Catholic Church of they left the country.
Easter is the second most important day of the Church year. The most important feast day is Pentecost. Had there been no Pentecost, when the Holy Spirit descended upon the Apostles, the Church would never have gotten off the ground. Pentecost is considered the birthday of the Church. So Easter and Pentecost are both important feast days in the Catholic Church.
The Catholic Church
Either McDonalds, or the Catholic Church.
The Council of Trent was the most significant event of the Counter-Reformation. It reaffirmed Catholic doctrines and practices, addressed corruption in the Church, and reformed the clergy. This council had a lasting impact on the Catholic Church and its response to the Protestant Reformation.