Floor
The floor is the underside of the car, and it produces the majority of a modern Formula 1 car's downforce.
Accelerating air underneath the car lowers the pressure there and effectively sucks it towards the track. Because it works with far less drag than a wing does, it is the most efficient downforce on the car.
That is also why floor damage is so costly. A visibly small tear can be worth a large fraction of a second, and unlike a front wing it cannot be changed in a pit stop.