First, Bascilica of St. Denis, home of the original Gothic arch and the first building to use stained glass in a large format. It is also the final resting place for 72 French kings and queens. Of course, for some the monuments only mark where they were before revolutionaries removed them.
Second, Sacré-Cœur, one of the most iconic monuments in Paris. The funicular ride to the top wasn't too shabby either.
The first came with lunch, the second with beer. Each is fabulous in its own way.