Go JetBlue to JFK or Delta to LGA. United flies to EWR, which is more of a pain in the butt to get into NYC than the other two.
I'd take the cheaper option first, and if they're the same, LGA is closer to midtown (but I believe only JFK has a fixed taxi fare). JFK is easy using the AirTrain to the subway.
I'd probably choose DL (LGA) if I had enough luggage to warrant a taxi, or B6 (JFK) if I was traveling light enough to take public transportation.