Formal:
* Crucial: Emphasizes extreme necessity.
* Essential: Highlights something indispensable.
* Significant: Points to something of notable value or impact.
* Vital: Stresses life-giving or absolutely necessary.
* Consequential: Emphasizes impact and result.
* Critical: Indicates a crucial point or something that could lead to failure.
Informal:
* Key: Suggests something vital to understanding or success.
* Major: Indicates something large or significant.
* Big: A simple and direct word for something of great importance.
* Huge: Emphasizes something extremely important.
* Necessary: Highlights something that is needed.
* Essential: Similar to "necessary," but with a stronger emphasis on being indispensable.
Specific:
* Urgent: For things that require immediate attention.
* Prioritized: For things that are given a high level of importance.
* Valuable: For things that are highly useful or have significant worth.
* Fundamental: For things that are the foundation of something else.
* Strategic: For things that are related to achieving a goal or objective.
Other alternatives:
* High priority: Emphasizes urgency and need for immediate attention.
* Top of the list: For things that are considered the most important.
* First and foremost: For things that are the most important consideration.
The best word to use depends on the specific context and the nuance you want to convey.