Madame de Poitiers
I think Ludivine Sagnier as Madame Diane de Poitiers is the best thing about this series. They're stretching with her age a bit. At one point someone comments that she is twice the age of the king, but the actress was only 42-43 when this was filmed. I suppose that worked when it was young teenage Henri before he was king, but she should have been in her 70's by the time he's been on the throne awhile.
Still, god bless her for that beautiful scene getting up out of the gold bath!