na-adjective, noun which may take the genitive case particle 'no'
most important; top priority
Used to describe the single most important thing, issue, or factor. Often appears in formal or business contexts.
安全が最重要だ。
Safety is the most important thing.
これは最重要課題です。
This is a top-priority issue.
重要 means 'important'; 最重要 adds the prefix 最 (most) to mean 'most important' or 'top priority'.
Compound of 最 (most) + 重要 (important).