The phrase "most critical reason" is correct and usable in written English.
You can use it when you are talking about the most important reason for something. For example: "The most critical reason for implementing the policy early was to ensure employee safety."
The most critical reason has been resources.
The most critical reason Mr. Hogan avoided the range, however, was wind direction.
The most critical reason is mianzi, or "face," as it is usually translated in English.
Perhaps the most critical reason for studying collective human dynamics under emergency/panic conditions is the lack of complementary data to develop and validate an explanatory model.
This editorial outlines only a few of the most critical reasons for why vouchers matter.
The sharp deterioration in the intrinsic value of the dollar since 2003 is a critical reason most people ended up economically treading water or falling back.
For this reason, the most critical concern in the maintenance and operation of strategic weapons systems was to provide certain and secure communication between civil and military commanders and to provide "backup" computer and other facilities wherever failure of a component could have lethal consequences.
For this reason, the most critical issue for the next-generation C-RAN system is to develop effective resource allocation algorithms [5].
There are a number of reasons for this but we believe one of the most critical barriers is not understanding the needs of government organizations.
Ultimately, it is the human cost that is most immediate and most critical, and it is for that reason I have made this appeal.
We reasoned that the most critical genetic components of the Treg lineage identity would be genetically and epigenetically conserved in mouse and human and additionally be Treg lineage-specific in both species.
