It may be unfair to say any one position is more important then the others, but you could make a case for linebackers being the heart of a defense.
To play the position, you ideally need good size and a physical nature to take on blocks and play near the line of scrimmage. You also have to have the athleticism to be able to move around in space and play in coverage.
It is also a position that demands a good mental capacity of the game and a team's particular defense. It is hard to cover up flaws when you line up at the linebacker position.