Victor Hugo is my favourite 19th century French writer.
'Les Miserables' and 'Notre Dame de Paris' are good, well told stories based on old themes, redemption and love respectively. Both are dealt with in depth and detail through the different points of view of several well drawn, lifelike and convincing characters the reader can empathize with, or be genuinely repelled by.
Historical adventure stories by Alexandre Dumas pere are amongst the best known in all of literature,Stendhal's realism was ahead of its time,Zola introduced naturalism in a series of novels about the adverse effects of industrialization, and Balzac's influence on both contemporary and future writers was and is important.
All in all, 19th century French literature at the top end is high quality, has a wide variety, and is hugely enjoyable, moving, and thought provoking to read.