So when, exactly, is the best season to enjoy Paris? The answer depends on how you'd like to experience it.
Spring is a great time for Paris. The flowers in the parks will be in full bloom, the weather will be mild, the atmosphere will be relaxed, the views will be at its finest, and everyone wants a share of this. Yes, I'm talking about thousands of tourists who will flood into the French capital during springtime. Queues will be longer, air fares will be higher, and reservations will have to be made. However, the upside of this is that since it's the high season, most shops and businesses will be open, and the city will be rather lively.
Summer is probably the most popular time for the visitors. There will be more tourists than Spring everywhere in the city. It's not ideal for those who wishes to experience the Parisian ways as well, since large numbers of the locals would've left the city for vacations in the likes of Marseille or Cannes. Many shops, from boulangeries to glaciers, will be closed. The weather can be very hot (around 30 degrees), and then become much colder and wetter on the next day (luckily, the Météo-France is usually accurate). It's not all bad, though. The sunny days are absolutely amazing, with locals lazing around the the parks and having picnics by the Canal Saint-Martin. Your photos would look amazing, too. Some also say that this is the most tourist-friendly season, and there will be a lot of free open-air activities ranging from the Paris Plage to the open air film screenings.
Free concert at the Parc de la Villette
Fall is for those who want a quieter time in Paris. There will be much less tourists and the locals will be busy with returning to business. Prices will drop, but so will the vibrancy of the people, and the days will be shorter and colder. Some, including myself, would appreciate the calmness though. It allows us to see the city in a different light, Paris in itself, without distracting crowds of people. It's a great time for visiting galleries for that very reason as well.
Winter will be cold, obviously. This could make it more difficult to walk around and explore the city; the days will be even shorter and some shops will be closed, but there is a certain charm of Paris under the snow. It's terribly romantic. Also, the holidays decorations and festivities are brilliant.