Formal:
* Essential: Emphasizes necessity.
* Crucial: Highlights criticality and potential consequences.
* Primary: Indicates priority and main focus.
* Fundamental: Emphasizes foundational nature.
* Paramount: Stresses highest importance.
* Preeminent: Points to superiority and outstanding significance.
* Cardinal: Suggests central and core importance.
Informal:
* Top: Direct and simple.
* Key: Highlights importance in relation to a specific outcome.
* Vital: Emphasizes life-sustaining or critical significance.
* Critical: Stresses urgency and potential consequences.
* Big: Informal and emphasizes size or scale of importance.
* Number one: Casual and straightforward.
Descriptive:
* Most valuable: Focuses on worth or benefit.
* Highest priority: Emphasizes ranking and urgency.
* Central to: Highlights importance within a specific context.
* Undisputed: Stresses that the importance is not debated.
* Unquestionably: Emphasizes that the importance is not in doubt.
Phrases:
* Of paramount importance
* The cornerstone of
* The driving force behind
* The key to
* At the heart of
* The most critical factor
Remember: The best word or phrase to use depends on your specific context and desired tone.