of great importance
cardinal - central
Something that is ‘cardinal’ is of central or vital importance that should not be broken, avoided, or ignored. Despite that, the term can be used in a humorous way or simply for effect.
When I’m working from home, the cardinal rule is that no one in my family is to disturb me until I’m finished.
Having coffee with cream is a cardinal sin according to Chris! He likes to think of himself as a connoisseur.
Completing the order before the deadline has been my cardinal concern this week.