which food or drink must be tried in France?

This a good credible source
http://www.travelandleisure.com/articles/the-25-dishes-that-define-france

The cheese, the cheese, and the cheese! From the earthy brie to the creamy roquefort (which many think leads to prolonged life span), there simply is no other place on this Earth you can savor cheese like the French. Bon Appetit!

Pastries for sure.

A few years ago, there was an article in The NY Times describing five quintessential dishes and the cities to visit in which to enjoy them. Here's a link to the article. After reading it, I took a trip visiting the cities and delighting in the recommended meals. It was Le Tour de France Gastronomique. Starting with choucroute garnie in Strasbourg, then heading south to Lyon to savor quenelle de brochet. Next to Marseille to splurge on authentic bouillabaisse. Next stop was for cassoulet in Toulouse followed by crepes and galettes in Brittany. I always asked for recommendations of local wines to accompany my meals and was never disappointed. Other foods to consider include fois gras, duck, steak tartare, moules et frites. The list is endless.

Cheese and pastries as mentioned, also:
Bread, sausage, crèpes, chocolate, boeuf bourgignon, coq au vin, cassoulet, home-made soups, Berthillon ice cream,
camembert with sweet butter on baguette (sandwich - at cafés or bakeries), quiche, wild strawberries, white asparagus, citron pressé (at cafés), real hot chocolate...

The five items article in the NYT is by a friend of mine! : ) In fact, it's derived by a book where she went around France and delved into quintessential dishes for a year, one for each month. Yum yum.

For drink I'll say a kir royal -- a glass of Champagne with creme de cassis (blackcurrant liqueur) added (they also make them with all sorts of other fruit liqueurs) -- you can also have with "regular" white wine, that is just called a kir.

And don't forget couscous, you'll find excellent couscous!!

".....there simply is no other place on this Earth you can savor cheese like the French....."

Au contraire!

There is more variety of cheese in the UK than in France (they have lots of regional variations of the same cheeses but the UK has more distinct varieties, 900 varieties in the UK compared to 400 in France). In blind tastes British cheese consistently out performs many French and other European cheeses and some of the top fromagerie's in Paris struggle to keep up with demand.

Thank you for your sharing. Now I can't wait to savor them!

This may not be what you had in mind, but I love Schweppes Agrume (that's citrus - like Schweppes Lemon but with a mix of citrus flavors). I've never seen it any where but France, and on my last trip, the supermarkets had it in a diet version - heaven. If, like me, you enjoy carbonated beverages, it's a must-try.

Moules Frites! ( mussels and fries). Had them 5 days in a row one year. Leon de .Bruxelles in Paris is a big and bustling restaurant (several locations) with nice big servings of moules frites.

La Pigeon, delicious and nutritious! A Quenelle, if you are in Lyon. I've had several variations of fried fois gras and fried figs, sometimes together, as an entree.

There are lots of Michelin restaurants in France. you should look it up on google. Also Escargot and wine are always a right choice in France!

Cheese, crepes, and the absolutely wonderful bread, especially the croissants - my lips feel greasy just thinking about how buttery they were. As for wine, I never had a bad glass of wine when ordering the house whites or reds, they were always very good.

Try a roast chicken with croutons from a local food market! We also purchased some of the best produce and fruit at the colorful markets! Have you had any Macaroons? They are amazing. Great chocolates also! Order Coq Au Vin, absolutely yummy! Have the Baba au rhum for dessert! I could go on and on.... Have fun!