I know there are many idioms for “masterpiece”, but there is one French idiom I came across that I am trying to recall. I believe it literally means “life’s work”, and it may have “morte” in the phrase…kind of a backward use of the word that refers to death! To save you all some trouble, I know it is not raison d’etre, magnum opus, joie do vivre, or tour de force. Still, maybe someone in the SD audience can take a wild guess at the phrase that pays? Presently, it’s elusive with a certain…je ne sais quoi!
Pièce de résistance?
Magnum opus is Latin, and I think we can be sure it’s also not Tour de France or Soup du Jour. 
I was going to suggest chef d’oeuvre, but I see someone else already has.
In one, with the slight nitpick that it’s technically spelled *œuvre *- but it’s not even a character on azerty keyboards and I had to look up the ASCII code :o. In any event, even in written form I’d wager most French speakers forgot all about their "o, e dans l’o"s :p.
Taken literally it means something like “topping/crowning [of one’s] work”. A great, but not **the **best work of an artist would be an **œuvre majeure.
Yeah, French speakers forget to pronounce half of the vowels anyway, why would you need another one? (besides confusing even more the pesky foreigners trying to learn French 
)
IIRC, Italian also has a word with a very similar meaning (capolavoro). I don’t know about Spanish or Portuguese.
Obra-prima in Portuguese.
As every non-Spanish speaking American who thinks he knows some Spanish knows, the equivalent word in Spanish is “El Masterpiece-o”.
Dear boy that is the heart, soul and function of French grammar and spelling. All of it.
Prièce, n’est-ce pas?
Nope, it’s pièce.
“I’m taking French lessons. Today we studied final consonants.”
“You don’t say!”
<badump-TISH>
I find that I can speak passable French by Speaking Spanish and making sure I drop the last syllable and slur the rest of it together.
I’m fluent in both…and you’re about half right.
BONUS: Put a couple of marbles in your mouth and you’ve got Portuguese covered as well. 
Ug! It’ll come to me one fine day…years from now. In the meantime, que sera, sera! (And, yes, I realize “magnum opus” is Latin.)