Key West, Florida is the farthest incorporated US city from Fargo, North Dakota.
Fargo (46.87° N, 96.79° W) is near the geographic center of the continent; Key West (24.56° N, 81.78° W) is the southernmost city in the contiguous US, maximizing the diagonal.
Honolulu (~4,500 mi) is farther but not in the contiguous 48.