First of all your question is wrong because you state that there are FIVE points we must examine and then you ask which of the SIX do we believe has the greatest influence.
I feel that illumination has the greatest impact because it is Ruach Hak-Kodesh that speaks to us and tells us if we are going in the right direction or not. This can also be a hindrance if we do not listen to Ruach Hal-Kodesh or we are trying to study in an environment that has so much noise or distractions that we can not hear him.