The hormone most associated with the fight or flight response is

Adrenaline.

Explanation:

The fight-or-flight response is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival.

This threat to survival generates a reaction in the nervous system that leads to a production of different hormones, especially noradrenaline and adrenaline.

Adrenaline is the body's activator, and is released in response to anxiety, exercise, or fear.

Therefore, the hormone most associated with this response is the adrenaline.