While it would help to narrow location a bit and give us a price range, I am assuming you are looking for something high end and geared to impress, which during the week can actually comprise one of the great New York bargains, since many of the top restaurants do a fixed price lunch which can actually be quite reasonable. As few midtown suggestions if you want to go high end:
Marea: 240 Central Park South. Amazing Italian seafood, they have a 45 dollar pre fixe lunch, which includes an appetizer and main course. Among the choices is a fusilli with bone marow and octopus which is one of New York's can't miss dishes, and you have the benefit of the bargain priced lunch.
Hakkasan New York: 311 West 43rd. While I do not usually recommend very fancy and high priced Chinese food, as it is usually dumbed down for western taste, Hakkasan is a very high end, elegant London based group which, for fancy Chinese geared for Western taste, is amazingly good and in a very cool setting.
Benoit: 60 West 55th Street. Formerly the relocated site of Cote Basque, and now owned by the famed Alain Ducasse, this is as close to a high end Parisian bistro as you can get in New York. Lunch can be suprisingly reasonable, with a $35.00 three course pre fixe.
I of course have my personal list of secret lunch spots, but if I told you I would have to kill you.
Bon Appéti