What is another way to phrase the expression highly urgent?

Here are some ways to phrase "highly urgent" depending on the context and level of formality:

Formal:

* Critical

* Time-sensitive

* Of paramount importance

* Requires immediate attention

* Must be addressed with utmost priority

* Expeditious

Informal:

* A rush job

* Top priority

* Needs to be done ASAP

* Can't wait

* Urgent, urgent, urgent!

More specific:

* Life-threatening (if it's a medical emergency)

* Time-critical (for deadlines)

* High-priority (for tasks)

The best way to rephrase "highly urgent" depends on the specific situation. Consider who you're talking to, the topic, and the tone you want to convey.