A word that means 'most important'?

I tried to find a single word that means "most important", but I couldn't. I want it to be able to express what's missing below:

If you get hurt, the _ thing to do is to stay calm.

It would need to describe something as being the absolute, single most important thing as opposed to just very important.

I did find the word "quintessential", but I think that word also has another meaning which is used more frequently.

I thought about making up my own, but I couldn't find a prefix that means "most".

I feel kind of silly, but I think I have been looking for a word based on a concept that doesn't exist in English. I was looking for a word that could never be used to describe two things as both being the most important. I think some of these words, like 'imperative,' express necessity as opposed to importance, but I may be wrong. The word 'key' expresses importance, but it, like 'important,' could be used to refer to multiple things. The two most important players are Sam and Ashley. The two key players are Sam and Ashley. I was thinking of a word that would describe importance as 'best' describes 'goodness,' but I'm realizing that that doesn't really make sense because even the word 'best,' though I think it tends to often describe only one thing, is often used to describe two things. The two best players are Sam and Ashley.

I guess, with the superlative, if you take any adjective and put the noun it's modifying in a singular form, it implies that that noun is the single strongest possessor of that adjective. I could just say "The important thing to do is to stay calm," and I think that would imply that that is the single most important thing to do because the word 'thing' is singular.

I think the fact that the same word can be used to describe the single strongest possessor of a trait (the tallest person) and multiple strongest possessors of a trait (one of the tallest people) kind of rubs me wrong. If anyone's curious, I was thinking of inventing the word 'monobest' or 'unibest' to mean single best. This way, you couldn't say "one of the monobest" because 'mono' means one, and something can't be both both multiple and one. Similarly, 'monotant' or 'unitant' could mean single most important (I've shortened the word 'important').

Thank you all, and sorry if this question was misleading.

"Paramount" might be the word you're looking for.

E.g.

The paramount thing to do is go back to your family and tell several of them what has happened.

According to context, you can also do pretty well with the colloquial "number one:"

I thought the number one thing to do is to not join. That way they can't have access to all your personal info including email addresses...

But the word that works best for your example is a simple "first."

If your baby has been hurt, the first thing to domis is to get immediate medical attention.

If you get hurt, the first thing to do is to stay calm.

How about crucial: "Extremely significant or important: a crucial problem; vital to the resolution of a crisis; decisive."

critical: Indispensable; essential.

essential: Basic or indispensable; necessary

vital: necessary to the continuation of life; life-sustaining:

It was critical that he stop them for if he did not the battle was sure to be lost.

If you get hurt, the primary thing to do is to stay calm. Also foremost or paramount.

foremost would fit.

adv. So as to be most important.

adj., adv. first in place, rank, importance, etc

There is the phrase "first and foremost" also, that emphasizes this:

first to be dealt with and most important.

First and foremost, I think you should work harder on your biology. Have this in mind first and foremost: Keep smiling!

Note: You can check example usages in Google Books as well.

In the context If you get hurt, the --- thing to do is to stay calm, you can use key.

In other contexts, prime, preeminent, or vital can be used.