For example, the full name of a French mathematician is Guillaume François Antoine, Marquis de l'Hôpital. Should I call him Monsieur de l'Hôpital, Monsieur l'Hôpital, or Monsieur Antoine?
I don't think he will mind how you address him - he's been pushing up the daisies for some time now.
In general I would not worry too much about French aristocrats, or the need to address them.
In the late 18th century, the French had gotten so upset with their aristocrats, they organized a little revolution, effectively doing away with the concept of aristocracy. Even the king was addressed as citoyen Capet before they separated him from his head.
Chances are that if you meet any Frenchmen who happens to have an aristocratic title today, they will (also) go by a "normal" last name, and you would be expected to use that name.
When you refer to the said mathematician, he's usually known simply as L'Hôpital.