The greatest Mitzvah is Tzedakah Charity.
“G‑d’s kindness” is drawn down through man’s “arousal initiated from below.” It is thus the coin that one gives a pauper that grants the giver the gift of “beholding G‑d’s face” — the internal aspect of G‑dliness — during prayer. In this way, man’s kindness and tzedakah elicit G‑d’s kindness and tzedakah.
However, the following must be understood: Since the Divine illumination must inevitably result from G‑d’s attribute of kindness and His tzedakah, why is man’s service necessary at all?
The Alter Rebbe answers this question by explaining that parallel to the above attribute, there also exists a Divine attribute of severity and contraction, that seeks to limit and screen the diffusion of the G‑dly light. It is man’s practice of kindness and tzedakah that ensures that the attribute of severity and Gevurah will not hinder the flow of Divine radiance that is to be revealed to him during prayer. Cf. Tikkunei Zohar 69:105a. Zohar II, 175b