I recently became curious about the most geographically extreme airports in the USA, and produced this list, with some interesting notes.

Where the most extreme airport is private, I also list the most extreme public airport.


Most Western Airport
The most Western Airport in the US is an interesting contest between the Lower 48, Alaska and Hawaii.

In the Lower 48, its Quillayute Airport, formerly a state airport.
It may now be best known as the setting of the popular Twilight novels, in and around Forks, WA

Alaska's most westernly airport is a Coast Guard Station on Attu Island, it is just 65NM from the International Date Line.
The most westernly public airport is Adak Airport on Adak Island, nearly 400 NM further east.

Hawaii's most western island with airports is Kaui.
Barking Sands is on the western shore, but is closed to the public as part of the Pacific Missile Range Facility.
In addition, the Sectional Charts warn of extreme Electromagnetic radiation for 8000 feet around the facility, which could "produce harmful effects to personnel and equipment".

Port Allen, about 13NM to the South East, is the most western public airport.

Lower 48:
UIL, Quillayute Airport, outside Forks, WA
47D56'11.70"N 124D33'45.48"W

Alaska:
Private: ATU, Casco Cove, Coast Guard Station Airport
52D49'57.15"N 173D10'31.99"E

Public: ADK, Adak Airport
51d52'40.67"N 176D38'45.71"W

Hawaii:
Private: BKH, Barking Sands Pmrf Airport
22D1'21.99"N 158D47'6.21"W

Public: PAK, Port Allen Airport
21D53'49.00N 159D36'11.40"W


Most Northern Airport
Most of the US Border with Canada lines along the 49th Parallel.
However, due to a treaty anomoly, the Northwest Angle, in northern Minnesota, is quite a bit above the 49th.
It only has one small airport, which is private. But at 45North, it is the most northern airport in the Lower 48 states.

After that, many small airports are right along the 49th parallel and the border, so its hard to pick a clear winner.
But two stand out. Scobey Border Station is a joint operation between the US and Canadian Government, and just barely over the 49th,
so technically in Canada, but still listed as a US Airport.
Whetstone International is officially at 48,59'59.96", putting it closer than any other airport.


Lower 48:
Private: Northwest Angle, in Northern Minnesota
45D15'27"N 101D41'39"W

Public: 8U3, Scobey Border Station / East Poplar International Airport, Montana
49D0'0.09"N 105D24.03'
This airport is technically over the 49th parallel, and therefore in Canada,
but it is listed as jointly owned by the US and Canadian Governments, in Montana.

Public:
H28, Whetstone International Airport, in Montana
48D59'59.96"N 112D47'33.29"W
This open-to-the-public airport is just 4 hundreths of one second from the 49th parallel, and therefore the closest to the border.

Alaska:
BRW, Wiley Post-Will Rogers Memorial Airport, Barrow Alaska
71D17'5.60"N 156D46'6.90"W

Unfortunately, the Most Eastern and Most Southern airports are not as contentious.
There are clear winners in each category.

Most Eastern Airport
EPM, Eastport Municipal, Maine
44D54.61'N 67D00.76W

Most Southern
Lower 48:
EYW, Key West International, Florida
24D33'22.00"N, Florida Keys

Hawaii:
ITO, Hilo International Airport
19D43'12.95"N 155D2'54.49"W