In his last Rougon-Macquart novel, Le Docteur Pascal, Zola gave a recommended reading order, although it is not required, as each novel stands on its own. You can find the series in publication order here: Les Rougon-Macquart

Note: This recommend reading order isn’t mentioned in the most authoritative French editions. However, in the fifth chapter of Le Docteur Pascal the Docteur Pascal summarizes all the Rougon Macquart history and the English journalist Ernest Alfred Vizetelly derived a recommended reading order from it, affirming that it was Zola’s recommended reading order.

Les Rougon-Macquart may be read in any reading order anyway, as long as you start the reading with La Fortune des Rougon and end it with Le Docteur Pascal. Although most of the novels stand on their own, it would be better to read L’Assommoir before reading Nana, and to read Pot-Bouille before reading The Ladies Paradise.