depends on what you like.
if you crave bistro stuff, head to lyon. if you want to eat surprisingly well for cheap, take the back roads and eat at truck stops. i personally have a preference for the provence region which is much more mediterranean influenced. if you enjoy tripe, you will enjoy "pied et paquet" which is probably most popular closer to marseilles but can be found pretty much throughout provence. if you like olives, definitely try the tapenade. garlic - the aioli, served with boiled veggies, fish & other seafood.
if we could get sandwiches in the US in supermarkets like the baguettes they sell in supermarkets everywhere, i could eat those for lunch everyday and be a happy person.