The piece de resistance of something is the most important part. You can describe your favorite sculpture in a museum as the piece de resistance. Feel free to say it with a French accent. Oui oui!
This French loaner has two typical contexts: at meals, where it denotes the most important dish, and in performances and exhibitions, where it denotes the highlight. The piece de resistance of an antique toy collection might be an original 1959 Barbie doll, for example. The literal translation is what you would guess — piece of resistance — and raises the question of who or what is actually doing the resisting.