December 23 and January 1-2 will be very tough dates for award availability.

In some ways, the obvious carrier would be AR, but it doesn't have a very good reputation for service. They fly nonstop from JFK to EZE, but then most flights would require you to switch to the domestic Buenos Aires airport. I'd start by looking for connections that avoid Buenos Aires, perhaps using TAM or GOL from Brazil, but you can have airport switches there too. BTW, AR is a DkyTeam member, so you could also do LGA/JFK/EWR-ATL-EZE on DL.