I consider the DOWN command the most important of the 3 positions. The positions are DOWN, SIT and STAND. In basic obedience a dog is required to know two, SIT and DOWN, but if we take it further, we teach the dog the STAND.
The DOWN command is critical because it is a completely “disabling” command. In other words, the dog is out of the range of movement, unlike in SIT and STAND where the dog is ready to move much more quickly. That is why I opt for a down when I want a dog in total control.
See: online dog training