Fastest is taxi. Flat fare of $45 + tolls (about $6) + tip (approx 10%+).
Also good is New York Airport Service Express Bus, $15. Should take about 75 minutes. The bus to Port Authority stops first at Grand Central Terminal. Leaves every 20 or 30 minutes.
Should be no problem to make 6:00 bus.
Fastest is taxi. Flat fare