Best train station in 5th to Giverny

You do not have a choice: trains to Vernon-Giverny leave from Gare Saint Lazare, which you can access by metro line 7 to Chaussée-d'Antin-Lafayette followed by a 10-min walk, or get off one stop before at Opéra and take line 3 to Saint-Lazare.

Edit: also, buses 21 and 27 go direct to gare Saint Lazare from near the bottom (north end) of Avenue des Gobelins. It is a nice ride but travel time is less reliable than by metro, so not ideal if you have a train to catch!

Thank you