This is the 4378th most frequent Russian word.
важнейших
Most important.
The word 'важнейших' is used here as a superlative adjective modifying 'задач' (tasks), indicating its highest importance.
Защита окружающей среды является одной из важнейших задач современного общества.
Environmental protection is one of the most important tasks of modern society.
In this sentence, 'важнейших' describes 'принципов' (principles) as being of utmost importance, crucial to the subject of discussion.
Важно выделить несколько важнейших принципов управления.
It is important to highlight several most important principles of management.
Here, 'важнейших' is applied to 'вопросов' (issues), again emphasizing their position of supreme importance.