If the OP actually means "downtown" as in Lower Manhattan (Wall Street) then EWR is arguably the most convenient. Otherwise, LGA is hands down the closest and easiest choice. JFK is a distant third in terms of convenience and cost of transport to the city - and the AirTrain is a joke, even for me, a hardcore believer in public transport to/from airports. Fly DEN-LGA and grab a cab, or if going to Grand Central consider the express bus option if money's an issue.