The Basilica of the National Shrine of the Immaculate Conception is one of Washington's most imposing religious buildings and the largest Catholic church in North America. Its scale, mosaics, chapels, and contemporary sacred art make it interesting well beyond a strictly devotional visit.
It stands in Brookland near Catholic University. Give yourself time to look through the upper church, lower church, and side chapels rather than treating it as a quick facade stop.