The best direction of a runway is along the direction of the longest line on the wind rose diagram. This is because the longest line represents the prevailing wind direction, which is the direction that the wind blows most often. Having a runway that is aligned with the prevailing wind will help to reduce the amount of crosswind that aircraft experience during takeoff and landing. Crosswind can make it difficult for aircraft to take off and land safely, so it is important to minimize it whenever possible.
The shortest line on the wind rose diagram represents the least frequent wind direction. This direction is not ideal for a runway because aircraft will experience more crosswind when taking off and landing into this direction.
The line clear of the wind rose diagram represents the direction from which the wind does not blow. This direction is not ideal for a runway because aircraft will not be able to take off or land into this direction.
Therefore, the best direction of a runway is along the direction of the longest line on the wind rose diagram.