The French monarchy was once one of the strongest in Europe. But since 1870, the country has been a republic. In the unlikely event that France were to restore its monarchy, the question of who would be king is not at all clear. The above chart (which you can click on for a better view) outlines the three main candidates, advocated in turn by the Legitimists, the Orleanists, and the Bonapartists.
The LEGITIMISTS argue that, since the Spanish branch of the House of Bourbon is now the most senior line in the family, the throne should pass to the most senior male in that branch -- Louis Alphonse, Duke of Anjou. (Note: Although he is the senior male in the Spanish House of Bourbon, Louis Alphonse is NOT the King of Spain. This is due to the fact that his grandfather -- Jaime, Duke of Segovia -- renounced his rights to the Spanish throne for himself and his descendants on account of being deaf.)
The ORLÉANISTS argue that the Spanish branch was cut off from the possibility of ever inheriting the French throne by the 1713 Treaty of Utrecht when King Philip V of Spain renounced his claim to the French throne for himself and for all his descendants. This would mean that the French throne would pass to the House of Orléans and its most senior male -- Henry d,Orléans, Count of Paris (a descendent of Louis Philippe I, the last king of France)
The BONAPARTISTS argue that the imperial family of Napoléon rightly replaced the Bourbons as monarchs of France and therefore the throne should pass to the most senior male in that family -- Jean-Christophe.
Finally, it is also possible that Louis XVII was rescued and that the boy who died in prison was actually someone else. Over the years, many individuals have claimed to be either Louis himself or one of his descendents but no claim has ever been substantiated.
Vive La Republique!