obscure2k's Taillevent suggestion is excellent. For a memorable lunch, I'd also suggest Le Grand Véfour -- a serious foodie might say that Taillevent has more Michelin hardware, but Le Grand Véfour has a fabulous something-very-special atmosphere as well. Some years ago, I recall my first visit there filled part with anticipation and part with dread fear that I'd do something dreadfully gauche. I'd gotten dressed up in my best ill-fitting sport and slacks, and was maybe a bit overwhelmed by it all. When I ordered a melon and prosciutto appetizer, my incredibly welcoming (but still formal) waiter whispered, so that no one from another table might hear, that while the melon/proscittu was excellent, maybe I would enojy a soufflé that I'd not find anywhere else. His perception and kindness cut through all my nervousness, and it was clear sailing from then on.

But hey -- maybe you weren't looking for a place where you'd want to dress up? If that's the case, I'd suggest La Ferme St-Hubert for a Croque-Monsieur -- a grilled ham and cheese sandwich like none other that you've ever had. Ferme St-Hubert is known throughout the coty as a great fromagerie, and their Croque-Monsieurs are top-quality. It's a classic bistro-style restaurant with small tables (outside as well if the weater is ok), and for me a perfect place for a nice lunch with a glass or two of Brouilly. La Ferme is on rue Vignon, just about in the middle of a triangle formed by the Opéra, Madeleine, and the Printemps department store.