While Annecy is indeed a beautiful city, I was unaware that it had been voted "most beautiful city in the world", Star Lune...
Anyway, any of those cities would be worth a visit. Grenoble and Geneva (which as she accurately pointed out is not in France but neighboring Switzerland) would provide ample opportunity to explore the Alps, while Salon-de-Provence and Cagnes-sur-Mer are located in Provence, a region renowned the world over for its beauty and proximity to the Mediterranean...
Paris (where I was born and used to live) is of course one of a kind, and there's not much I could say about it which hasn't already been said by countless other people.
I don't know Tours or Caen, but I've heard Tours is beautiful. Caen, if I'm not mistaken, was badly destroyed during WWII (as were many French cities on the English Channel), though that doesn't necessarily mean it lacks appeal to visitors.
Lastly, I've never been to Nantes but I've been told by someone who did go that it was the most beautiful city she visited in France.
In short, I don't think you can go wrong with any of these destinations.