Depends how much you want to spend, and how big the party is.
Robicelli's holiday flavors are well thought out knockouts without equal.
Blackhound always has interesting holidays flavors.
Tribeca Treats has some interesting seasonal items, like the whoopie pies.
Sugar Sweet Sunshine is a good standby if you need something less refined, and half the price of Robicelli..but I would start with Robicelli.
170 2nd Ave, New York, NY 10003
Sugar Sweet Sunshine
126 Rivington St, New York, NY 10002
94 Reade St, New York, NY 10013
Wouldn't recommend them if they didn't.
Are you mostly focused on cookies?
Sweetery Truck and the Treats Truck are good for seasonal flavors.
Our Daily Bread has a Molasses cookie at the Union Sqr. farmers market. (check the schedule)
Dougboy Bake Shop has a peppermint brownie that's been singled out on another site.
Union Square Greenmarket
Broadway and E 17th St, New York, NY 10003
, New York, NY 10001
535 Hudson St, New York, NY 10014