* Who is a pronoun and should not be capitalized.
* Is is a verb and should not be capitalized.
* Your is a possessive pronoun and should not be capitalized.
* Favorite is an adjective and should not be capitalized.
* French is an adjective referring to a specific nationality and should be capitalized.
* Artist is a noun and should not be capitalized.
Therefore, the correctly capitalized sentence is:
Who is your favorite French artist?