Depends on when you're arriving and where from (as HPN might be an option for you, which would really be the easiest). If HPN's not an option due to schedule or cost, then I would say LGA would be easiest, and JFK would be second. EWR is a distant third in my book. But a lot depends on the time of day you're arriving. At off peak times, JFK isn't a bad option, nor is EWR. I can't speak for the car rental process at any of those airports though, as I've never done it. At HPN, the cars are right outside of the baggage claim, so that's very simple.