With an architectural design team consisting of Michelangelo, Donato Bramante, Carlo Maderno and Gian Lorenzo Bernini, St. Peter's Basilica is widely considered to be the ultimate example of Renaissance architecture.
Due to its immaculate Renaissance architecture, as well as being the purported location of the remains of St. Peter himself, the basilica is one of the most sacred sites in all of Catholicism. Even if you're not a practicing Catholic, there's simply no denying the jaw-dropping grandeur of this monumental building.