Hagia Sophia (Istanbul, Turkey)Famous for its massive dome, the Hagia Sophia is widely considered the best example of Byzantine architecture. It was the largest cathedral in the world for almost 1,000 years until the completion of Spain’s Seville Cathedral in 1520. Originally built as a church between 532 and 537 A.D., Hagia Sophia later became a patriarchal basilica, then a mosque, and is now a museum with rich interior details, including intricate mosaics and marble floor decorations and pillars.
Westminster Abbey (London)Traditionally the place of coronation and burial for the English monarchy, the Collegiate Church of St. Peter, Westminster, more commonly known as Westminster Abbey, is a mostly Gothic-style church located to the west of the Houses of Parliament. In 1998, Queen Elizabeth II unveiled statues of 10 20th-century Christian martyrs from around the world who are depicted above the abbey’s Great West Door; among those honored is Nobel Prize-winner the Rev. Martin Luther King Jr.
St. Mark’s Basilica (Venice, Italy)A prime example of Byzantine architecture, St. Mark’s Basilica is one of Venice’s most famous churches, prominently located on the city’s central St. Mark’s Square. Opulent in design, and boasting gilded Byzantine mosaics, the building once regarded as a status symbol of Venetian wealth and power earned the nickname Chiesa d’Oro (Church of Gold) beginning in the 11th century. Since 1807, the cathedral has been the seat of the Roman Catholic Archdiocese of Venice.
St. Isaac’s Cathedral (St. Petersburg, Russia)Built in the traditional Russian-Byzantine style, featuring a Greek-cross floor plan with a large central dome and four smaller domes, this church is the largest cathedral in St. Petersburg. It is located on St. Isaac’s Square and is dedicated to St. Isaac of Dalmatia, a patron saint of Peter the Great. Designed by French architect Auguste de Montferrand, the cathedral was built over four decades, from 1818 to 1858. During World War II, the cathedral’s impressive gold dome was painted gray to be less conspicuous to enemy aircraft.
Seville Cathedral (Seville, Spain)The largest of all Roman Catholic cathedrals, the Seville Cathedral is the world’s largest Medieval Gothic religious building, in terms of both area and volume, and possesses what is considered to be the largest main altarpiece in the Christian world. Sitting on land previously occupied by the Almohad Mosque, the cathedral was built to demonstrate Seville’s wealth and boasts a lavishly decorated interior, much of it adorned with gold. The cathedral houses a large collection of religious jewelry and paintings as well as the tomb of Christopher Columbus.St. Peter’s Basilica (Rome)Located within Vatican City in Rome, St. Peter’s Basilica is often regarded as one of Christianity’s holiest sites. In Catholic tradition, it is the burial site of St. Peter, one of the twelve Apostles who was, later, the first bishop of Rome. St. Peter’s is famous as a place of pilgrimage, as well as for its liturgical functions. During the 16th century, artist Michelangelo worked extensively on the building and designed its Campidoglio. In December 2007, a red chalk drawing for the basilica’s dome, Michelangelo’s last known sketch, was discovered in the Vatican archives.Washington National Cathedral (Washington, D.C.)Listed on the National Register of Historic Places, this Episcopal church, officially named the Cathedral Church of Saint Peter and Saint Paul, and designated by the U.S. Congress as the nondenominational National House of Prayer, is the sixth-largest cathedral in the world and second-largest in the U.S. In 2007, the building was ranked third on the “List of America’s Favorite Architecture” by the American Institute of Architects.
NB: I've omitted some entries.