Parroquia de San Miguel Arcángel

To quote from the Lonely Planet entry, "San Miguel's most famous sight is its parish church, characterized by its pink 'wedding cake' towers that soar above the town. These strange pinnacles were designed by indigenous stonemason Zeferino Gutiérrez in the late 19th century. He reputedly based the design on a postcard of a Belgian church and instructed builders by scratching plans in the sand with a stick. The rest of the church dates from the late 17th century."

Note that this is a parish church, not a cathedral, since San Miguel de Allende does not have a bishop. Nevertheless, it's as elaborate and impressive as many cathedrals.