St. Peter's Basilica in Vatican City.
Considered the most renowned work of Renaissance architecture, as well as being the largest church in the world, St. Peter's Basilica in Vatican City sits atop the sight where St. Peter, aka Simon, was (allegedly) crucified upside down until death for being a follower of Jesus. Regarded as being the chief apostle, St. Peter was also made the first Bishop of Rome, prior to his death. The church is still active to this day, with the current Pope presiding over various liturgies throughout the year, drawing in audiences of up to 80,000.
As an interesting side note, there are presently over 100 tombs located within the Basilica (many located underground); of which, 91 are Popes, with the most recent interment being Pope John Paul II on April 8, 2005.