It was either king Louis XV or his mistress de Pompadour.
https://en.m.wikipedia.org/w/index.php?title=Apr%C3%A8s_moi,_le_d%C3%A9luge&diffonly=true

By the way "Let them eat cake" was attributed to Marie Antoinette by revolutionaries to spur anger among the plebes. She also never said it. It was pure propaganda. But it worked like crazy.