In the Book of Tehillim, three psalms begin with the word "tefilah," and this is connected with Moshe, David, and "the poor man": "Tefilah le-Moshe ish ha-Elokim" — (Tehillim 90:1)
(The prayer of Moshe, the man of G-d) — there is no prayer like this among other people.
"Tefilah le-David" — (Tehillim 17:1)
(The prayer of David) — there is no prayer like this from any other king.
"Tefilah le-ani" — (Tehillim 102:1)
(The prayer of the poor man) — this is truly prayer.
The Zohar HaKadosh asks: Which of these three prayers is the most important? And it answers: the prayer of the poor man. This prayer is more important than the prayer of Moshe, more important than the prayer of David, and more important than all the prayers in the world! Why? The poor man's heart is broken and crushed. It is written: "G-d is close to the brokenhearted." The poor man always has complaints against G-d, as though he were arguing with Him, and G-d listens attentively and hears his words. All the Windows of Heaven open when the poor man prays, and his prayer precedes and delays the entry of all the other prayers rising upward, because his heart is broken. This is written: "The prayer of the poor man, when he "wraps himself" (delays it). Why is the word "ya'atof" (wraps himself) written, rather than (is wrapped)? The reason is that the word "ya'atof" also means "will delay." Thus, all prayers can be accepted only after the prayer of the poor man enters. And all the Heavenly Hosts ask one another: What is G-d doing? Where are His efforts directed? They are answered: With great passion, He unites with His vessels—that is, with people whose hearts are broken and crushed. No one knows what takes place during the poor man's prayer: his outrage, the way his tears pour out before the holy King—and in this is the poor man's passionate desire to unite with G-d. And G-d's passionate desire is that He accepts these tears shed before Him. This is the prayer that delays, because of which all the prayers in the world are held back. G-d said: "Let the other prayers wait, and let this prayer rise up to Me!" There is no need for judges to decide between us; let him personally present his complaints to Me—let him be alone with Me. And the poor man's outcry appears before G-d in his prayer itself, as it is written: "Ve-lifnei Hashem yishpoch sicho" — (and before G-d he pours out his prayer). Of course, before G-d! Moshe prayed, and during this prayer he was delayed for several days—because of the prayer of the poor man. David saw that all the Windows and all the Heavenly Gates were ready to open before the prayer of the poor man. And he saw that G-d listened more attentively to the prayer of the poor man than to all the prayers in the world. When David understood this, he presented himself as poor and afflicted, removed his royal garments, sat on the ground like a beggar, and offered this prayer: "A prayer of David: incline Your ear, O G-d, and answer me." On what basis? "For I am poor and afflicted." G-d said: "David, are you not a king who rules over mighty kings? How, then, do you present yourself as poor and afflicted?" David immediately redirected his prayer: "Guard my soul, for I am a chasid." David included every level: both that of the poor man and that of the chasid.
(Based on the Zohar on "Balak," Sulam, translated by Zvi Nisanzon)