It is highly personal which are the best French writers, since it is highly based on opinion. However, there are a few names which can pretty safely be listed as some of the greatest French writers of all time without much debate. The first is Marcel Proust who authored In Search of Lost Time.
Marcel's writing often have deep, and powerful phrases such as "Love is a reciprocal torture." Another author who often comes up during this topic is Albert Camus who wrote The Stranger. The Stranger discusses philosophical and existentialist topics and view points. These are just two of numerous great French authors.