Front wing
The front wing generates downforce over the front axle and sets up the airflow for the whole rest of the car.
Its second job is the larger one. The wing shapes the air heading towards the floor, sidepods and rear, so damaging it costs far more than the downforce lost at the front — the balance shifts and the car is compromised everywhere.
It is the most commonly replaced part in a race, because it is the first thing to touch anything, and the adjustable flap angle is the one aerodynamic change teams can make during a pit stop.