The title of Duc was the highest ranking title a nobleman could hold; originally there had been 6 Ducs of France who - even though their origin was in the Middle Ages - still had a function during for example coronations. Ever since the 13th century and up until the revolution several counties were elevated to dukedoms.
The story of the French dukedoms is a very complicated one so what you will find here are facts relating to the titles in use during the Ancien Regime.
These were some of the titles - to some are added a few facts. The little star means that the dukedom was merged into the royal domain and would after that often be more of a courtesy title than an indicator of power:
The first-born nephew of Louis XVI received this title immediately upon his birth.
Louis XV held this title in the short period before his parents and elder brother died.
Duc de Bourbon*
Duc de Broglie
During the Ancien Regime all three holders of the title were also Marshals of France
Duc de Bourgogne
Was for a time used by the Hapsburg dynasty but France laid claim to it sometime after 1703 - Louis XV's parents were the Duc & Duchesse de Bourgogne
Duc de Guise
The title continued even after the revolution and was not extinct until 1830
Duc de Lorraine*
Marie Antoinette's father was a Duc de Lorraine, and Marie Leszczynska's father received the dukedom as compensation for his loss of the Polish throne - after his death it passed to his son-in-law, Louis XV
Duc de Montpensier
The Grand Mademoiselle held the title of Duchesse de Montpensier
Duc de Noailles
The Duchesse de Noailles received the nick-named "Madame Etiquette" by Marie Antoinette
Louis XIV's brother was given the title of Duc d'Orlèans, his descendant would act as Regent for Louis XV
You might encounter the strangely vague title of Duc de France which is a reference to all Ducs within the area Île-de-France (where Paris and Versailles is located). Some Ducs were what is called Ecclesiastical peers which meant that they held a post in the church as well as the title of Duc.
So, what benefits came with this prestigious title?
Well, first of all a Duc automatically received the Honours of the Court and his Duchesse was entitled to be seated on a tabouret (a chair with neither arms nor back) in the presence of the Queen. During ceremonies such as baptisms, weddings, processions, funerals and banquets they had the right to walk immediately behind the Children of France and Princes of the Blood - which meant that they were next in line only to the royal family.
During a lit de justice they were seated above the councillors and were not required to kneel when speaking to the King. They could vote at every session of Parlement and whenever an issue of theirs arose it would be judged in the Grand-Chambre.