LaGuardia I think you can take an N/R train, but I think you need to take a bus to the subway. They have shuttle buses from midtown to LGA, so I’d imagine the other way around as well.
JFK - You could take a cab, but with the traffic around 6, you are better off walking. I would suggest taking the A train, it should be running express, and going into the city isn’t a problem at rush hour, just getting out. It should take about an hour and fifteen minutes.
Newark actually might be the fastest route. The Train goes straight to Penn station. IIRC it takes about 20 minutes. Make sure to check train times though, because 20 minutes isn’t squat if you have to wait 2 hours for a train.
Unfortunately I’ll be working and will miss the beer, but enjoy the city.
I may be biased, since I live close to Newark, but it’s probably your best bet at that time of day.
AirTrain Newark gets you from the terminals in Newark to the NJ Transit station: from there it’s another 20 minutes into midtown (Penn Station).
LaGuardia and JFK are serviced by express buses that get you into Midtown: the problem is the traffic you would hit that time of night… I’ve used these people to get to/from LaGuardia, never JFK.
It also depends if money is an object, or how early you get into Manhattan matters… that JFK flight does get you in earlier.
I used the train from Newark recently. It was easy and cheap. From the plane to the train took me about 15 min. (I had only carry on) The train runs every 3O min so you may have to wait that time or you may luck out and a train will be arriving when you get to the platform. Then from when it picks you up it takes another 20 min tops. I think the cost was something like six bucks. Plus the train from the airport to the NJ Transit train station is really cool and futuristic.