Izipo
Hardcore casual gamer
SHAME ON YOU then!
Yes, I feel it.

Actually, it's Pierre Koeing, made maréchal in 1984 (Posthumus).
Well, the previous ones (Tassigny, Juin and Leclerc) were all made Maréchal de France in 1952, but Juin was the only one still alive (the two others were Posthumus).
But anyway, it's not the correct answer... As Maréchal de France is not the highest rank that existed during the history of France.
This is why I said my answer was technically correct, posthumous awards don't count. (well they shouldn't count in this specific instance, so I can claim a win

And this is why I should think a bit more before answering. But to be honest, I had totally forgotten about Koening.
As for the other part... I'm still looking for that, it is indeed an interesting question.
EDIT : Okay, I think I've got it (not sure); I'll let others search for it for a while, because it can be fun and instructive. Btw, the French Wiki is a total mess, I'll probably create an account soon to rectify all the mistakes there (at least in the military history portal).
So, just to be sure, is the 'rank' you are looking for from the XI to XIX century ?