To echo the previous thoughts, this is completely speculative. In my mind, the rights of the accused, as embedded in the Constitution, represents the most important component of the criminal justice system. The idea of the accused having the standard of being "innocent until proven guilty" and the prosecutor having to meet an evidential burden of different types in attempting to prove the guilt of the defendant as well as the accused's opportunity to cross examine witnesses, to openly face them, as well as the examine the evidence presented against them in a careful and thorough manner all represent aspects of the judicial system that help to ensure a sense of fairness and theoretical equity.
There is no way to give you a factual answer to this question as it is a matter of opinion.
You could argue that juries are the most important component of the criminal justice system in the United States. They are guaranteed by the Constitution and are the ones who determine guilt or innocence.
On the other hand, the vast majority of cases never go to trial because of plea bargains. Because of this, you can argue that the prosecutors are the most important component because they are the ones who determine what plea deals will be offerred.