Formal:
* Ma douce dame: This is the most formal and poetic way to say "sweet lady." It literally translates to "my sweet lady."
* Chère Madame: This is a more common formal way to address a woman, meaning "Dear Madam." You could add "douce" before it to make it more affectionate: "Chère douce Madame."
Informal:
* Ma belle: This is a very common and affectionate way to address a woman, meaning "My beautiful."
* Ma douce: This is a more intimate and affectionate way to say "My sweet."
* Mon cœur: This means "My heart" and is a very loving way to address someone.
More playful:
* Ma petite chérie: This means "My little darling" and is a more playful and affectionate way to address a woman.
* Ma sucrette: This is a slang term for "sweet lady" and is often used in a playful or flirtatious way.
The best way to say "sweet lady" in French will depend on the context and your relationship with the woman you're talking to.