As controversy swirls around demands to remove the memorials to the Confederacy and its officers found in many US cities, it’s worth remembering that a lot of those statues “were cheap, mass-produced and exactly the kind of disgrace that [the great memorial sculptor Augustus] Saint-Gaudens condemned. With this in mind, we asked five specialists in American sculpture to select the pieces they regard as the nation’s truly great works.” (For better or worse, all five are in the Northeast Corridor.)