You know what it is? Glad you asked. No, not the Winged Victory or the Venus de Milo. Not the Code of Hammurabi, and not even the Vermeers although they, like all Vermeers are eye-popping.
No; I cast my vote for the Marie de' Medici cycle, Peter Paul Rubens' 24 billboard-size paintings that occupy their own room in the Richelieu Wing, second floor on the north side. It's a tic: I'm a sucker for neoclassical/rococo pomp--same reason that I like the Tiepolos that can put us immediately in touch with so foreign a world. Also, of course, because Rubens (like Tiepolo) is just good at what he does--dynamic, brimming over with vivacity and sheer brio. There is a distinction here: Tiepolo, you suspect, is in on the joke, but with Rubens it is just high spirits.
Also, again, the sheer absurdity of it all. Here is a woman never did a single earthly thing of note except throw a king, who returned the compliment on reaching his majority by chasing her out. Rubens, a consummate master at of knowing which side his bread is buttered on, went light on the domestic rancor and brought the story to an edifying reconciliation.
All of which is enough one more item: is there anything more suitable to serve as so grand a center of so grand a museum, in so grand a city, at the heart of a (formerly?) grand nation?