Definitely Cerdan. It's hard to assess his overall ATG ranking because his career was cut short due to his untimely death, but at the time when he was getting ready for his rematch with Jake Lamotta, he already accrued over 100 wins and had a big time victory already over an ATG in Tony Zale. Had he been able to continue on, who knows how high he would've catapulted through the ATG rankings. But he's definitely the best boxer France has ever produced.
Cerdan technically Algerian, but yea, Cerdan a legend no doubt. Basically fighting with 1 arm vs LaMotta, who knows how the rematch would have turned out and God knows how a Cerdan vs Robinson might have played out. Get goosebumps about that one, we might have lost a classic superfight or Robinson-Cerdan trilogy.
Between Carpentier, Cerdan and Thil imo. Carpentier is the obvious choice though, although his legacy is quite obviously tainted. Cerdan has the profile of fighting Zale and LaMotta, Thil has the profile of being involved in a horrifically fractured MW division (a bit like pre Hopkins). I can't think of anyone who can compete with those 3.