if travelling from Barcelona to Paris, and if you can only stop in one city, What would it be?
Well, laughing, to me it would depend on how many days you have in Paris as to what I'd want to do on the way....
We stayed in Nimes. It takes 3:30 to get there on the train.
Traveling by car or train? It would make a difference in my choice.
There are many choices, but my choice would probably be Lyon.
We will be travelling by train.
What do you want? What are your interests? Roman ruins, a taste of the south of France, Provence, art or history museums? What month?
BCN is a fairly recently built Catalan city. Paris is mostly 19C buildings and monuments and updated medieval. Lyon has a large mix. Nîmes is the south and has two remarkable Roman structures. Narbonne the same. Another stop on that line is Montpellier, a thousand year-round university city. Vibrant. Valence is another stop depending on your train. It's the northern tip of Provence before Lyon.
For 1-2 days, my top choice would be Nîmes, followed by Montpellier.
For 3 days or more, I would pick Lyon - or stick with Nîmes, with a day trip to Arles.