Sacre-Coeur, or Basilica of the Sacred Heart, in Montmartre. It was built during the Third Republic as an apology to God for the moral degeneration that led to France’s defeat in the Franco-Prussian War and the Paris Commune—and it was built at a high point of the city, dominating the skyline of most traditionally leftist neighborhood of the city, on the Left Bank of the Seine. French leftists HATE the cathedral (seriously, the history is fascinating), but it is the second most popular tourist destination in Paris after the Eiffel Tower. A beautiful building!