Every time of year is great for visiting France, for different reasons and regions.  If you want warm weather, spring and summer are the best option.  However, July and August have loads of tourists, and it is also the most common season for French people to use part of their 5 weeks of vacation time, so the beaches are generally packed.  The "shoulder season" (May-June, Sept) has fairly nice weather, but fewer crowds.  Those from the Northern US or Canada might even call it warm.

All things tourist-related (souvenirs, hotels, street vendors, etc.) cost MUCH less during the winter, except at the ski resorts, so if you don't mind rain or snow, you will save a bundle.  In July, I paid 5 Euros in Paris for a small painting that I bought 2 years later, in December in Paris (as a gift), for 2 Euros.  Hotels offer similar discounts because they want to remain full.  Winter also gives you many opportunities for skiing in the Alps and the Pyrenees.