Formal:
* Bien (literally "well") - This is a general and very formal way to say "doing well."
* Très bien (literally "very well") - This is a more emphatic and formal way to say "doing well."
* Je vais bien (literally "I am well") - This is a more personal and polite way to say "doing well."
Informal:
* Ça va (literally "it goes") - This is the most common and informal way to say "doing well" in French.
* Bien (literally "well") - This is a more informal and general way to say "doing well."
* Pas mal (literally "not bad") - This is a more casual way to say "doing well" and implies that things are going alright.
Examples:
* "Comment allez-vous?" (How are you?)
* "Bien, merci." (Doing well, thank you.)
* "Comment vas-tu?" (How are you doing?)
* "Ça va bien." (Doing well.)
* "Comment ça va?" (How are things going?)
* "Pas mal." (Not bad.)
The best way to choose the right phrase depends on the situation and your relationship with the person you are talking to.