We've all heard of the Old North Church, made famous by Paul Revere, but one of my favorite buildings in Boston is the Old South Church. The copper-clad cupola was inspired by the cupola atop the Basilica of St. Mark, in Venice. The mix of ornate details and materials—along with the church's commitment to social justice and equality—is what makes it stand out to me.
When visiting, be sure not to miss the interior, if it's open. You'll find this church to be just as beautiful inside, as out.