This is a bit of a tricky question. JFK is much further south than LaGuardia, making it much closer to downtown Manhattan, north/south-wise. But JFK is also much further east than LaGuardia, making LaGuardia technically closer as the crow flies (meaning, if you were to draw an imaginary, direct line between the two).
Basically, if you were to draw a straight line west (northwest, actually) from LaGuardia to Manhattan, the line would hit upper Manhattan (at around 116th Street or so). If you were to draw a straight line (north)west from JFK to Manhattan, the line would hit lower (downtown) Manhattan, a little bit south of 14th Street. But since JFK is much further east, the drive in miles from LaGuardia to downtown Manhattan is still a bit shorter than the drive in miles from JFK to downtown Manhattan.
Newark Airport is closer, but only by a few miles. Newark is roughly 8 miles from Lower Manhattan as the crow flies (meaning, if you were to draw an imaginary straight line between them on the map), while JFK is roughly 12 miles from Lower Manhattan as the crow flies.
It is all relative to your eventual destination. JFK is quite distant from Manhattan, for example but closer to Queens. Newark is not as close as La Guardia. Each airport has advantages and disadvantages depending on your needs and wants. So, what are your needs or wants?
Closer than what? JFK is on Long Island, about 20 or so miles from Manhattan.
Newark
Laguardia
It is little more than 5 miles between LaGuardia Airport and Harlem as a straight shot (in Manhattan). If you are specifically asking for Midtown, it's between 6 and 6.5 miles (9.7 to 10.5 kilometers) from LaGuardia Airport to East 59th Street, driving west on the Triboro Plaza, over the Robert F. Kennedy Bridge, and then down the East Side of Manhattan.
The airport code for Downtown Manhattan Heliport is JRB.
Not far. It's between 6 and 6.5 miles (9.7 to 10.5 kilometers) from LaGuardia Airport to East 59th Street, driving west on the Triboro Plaza, over the Robert F. Kennedy Bridge, and then down the East Side of Manhattan.
The nearest: Upper Manhattan to LaGuardia Airport is 16 min (8.3 mi) via I-278 West.Upper Manhattan to Newark Liberty International Airport is 29 min (24.0 mi) via I-95.Upper Manhattan to John F. Kennedy International Airport is 27 min (17.6 mi) via Grand Central Pkwy.
LaGuardia is closer. Both JFK and LaGuardia are in Queens. Newark is in New Jersey.
Newark Kennedy Laguardia Plattsburgh AFB
Either John F. Kennedy International Airport, LaGuardia Airport, or Newark Liberty International Airport.
The Hudson River - this answer is wrong. The Hudson River is on the West side of Manhattan. The East River is on the East side of Manhattan. Laguardia airport is actually on Long Island. The East River still flows past the north side of LI where LGA is located.
It doesn't matter. All three of the major airports in the New York City area -- LaGuardia, JFK and Newark -- are roughly equidistant from Manhattan. LaGuardia is the closest by maybe five miles.
It is about 4.5 miles (7.24 kilometers). Brooklyn and Queens both occupy the western end of Long Island, and Queens is on top of Brooklyn. LaGuardia Airport is at the northern edge of Queens. The northern edge of Brooklyn is roughly 4.5 miles/7.24 kilometers south of LaGuardia.
The nearest major airport is Chicago Midway International Airport (MDW / KMDW). This airport has international and domestic flights and is about nine (9) miles by car from the center of town. Chicago O'Hare International is about 19 driving miles from the center of Chicago.
LaGuardia Airport is located in northern Queens and south of the Bronx. It is a short bus/subway trip to most of the city. Beware that there are more flight delays started at LaGuardia than anywhere else in the country.