Well, if your garden MUST be in France, I would suggest Monet's Garden. I found it quite charming, perhaps it's not your cup of tea ~ oops ~ that'd be England. I also enjoyed the gardens at Versailles.

If you are in Lyon see the Parc de la Tete d'Or - there is a 5 hectare (about 12 acres) rose garden that is great. It also has a small botanic garden there, but the rose garden is the thing to see.

Try to get to Vaux le Vicomte - it is the epitome of French garden design.

In Paris - jardins de Luxembourg - nice public park; les Tuileries; the gardens at the Musee Rodin are a nice little treat; botanic gardens by the natural history museum are also good.

Of course many of the chateaux in the Loire valley have some nice gardens with them.