Train from Avignon to Barcelona

What do you mean by best? Amount of time? Cost? Number of connections. You can check schedules at www.bahn.de/en. Choose Barcelona Sants as your arrival station. Avignon has two train stations. Avignon TGV makes sense if you are returning a rental car before the trip or not staying in Avignon itself. If you leave from Avignon Centre, you will have take a shuttle to the TGV station to get the train to Barcelona. To buy your tickets, go to captaintrain.com. Tickets generally go on sale 90 days before travel date and the less expensive tickets (usually these tickets are non-refundable and non-exchangeable) sell out faster.

I can't give you any particulars, but I've always wanted to take the "little yellow train" over the Pyrenees into Spain. The scenery is supposed to be sensational and the train small and wonderful. Just Google "little yellow train" to get you started - or maybe some RS travelers have taken it!