A word that means 'most important'?

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.

"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.

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!

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

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

The area to explore here begins at the word superlative..

..a word which is often used as the title of awards given to groups of people at era ends, such as at the end of high school for yearbook purposes; in that context, "superlative" awards may contain categories such as "most likely to succeed" or "best hair" or "least likely to harm a fly" etcetera.

Basically, the ideation implied by superlative involves seeking out, for purposes of identification, the most extreme end of a spectrum involving two trivial opposites such as tall and short or desirable and undesirable.

All other answers supplied began at this concept and ended with a trivial selection by the author of the response.

I would go with BEST.

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

But also, PRINCIPAL, MAIN, or INDISPENSABLE could work.

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

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

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

You could use crux of the matter to be a little fancy.

But I would just go with number one.

Imperative.

It is imperative that you stay calm.

Thought someone would have put this, but the first word that came into my head was "priority".

Priority

highest or higher in importance, rank, privilege, etc.: a priority task.

I can't yet comment, or upvote, but the single best answer is imperative.

Imperative.

It is imperative that you stay calm.

as suggested by drukenwagoner.

Quintessential is normally used to define something as representing the 'purest' form, e.g. "quintessentially evil"

Critical (medica) is a strong contender, as a thing that is critical must be done before anything else, otherwise the following actions will fail, though usually associated with an action, not a state of mind.

First (Nourished Gourmet) and foremost (Spehro Pefhany) are both ordinal statements, indicating that a transient step should be taken, where as the state of mind referenced by the OP is a persistent theme.

Crux of the matter doesn't fit as the OP is not trying to produce a clarifying statement of a convoluted argument or conversation, there is no "point" to be restated for clarity.

Indispensable is a statement of necessity not importance. Things can be indispensable yet trivial, due to their availability.

My own suggestion would be vital, as in "it is vital your organs remain connected and inside your body".

Imperative - of vital importance, crucial

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

Most important and best are both superlatives, the fact that the former consists of two words does not lessen its greatest in quality. Nothing else which may be suggested is more important than staying calm. On the other hand, "monobest" as you described it

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.

might be potentially harmful.
Imagine being seriously injured, "the "monobest" thing is to keep calm", mono- implies that nothing else is necessary, the act of staying calm is the one thing to do. Why do anything else afterwards? Instead, there might be several things one could do in a similar situation; call an ambulance, bandage the wound, place an ice pack on the injury and so on.

You do not want to exclude other courses of actions, there has to be a list of some sorts. In other words, because the superlative form, most important, is used for comparing one thing with every other thing in that group, it fits perfectly well in your sentence.

Good suggestions here already, especially "critical", "paramount", "foremost", and "primary" IMHO.
I'd just like to add "pivotal" to the growing list.

#piv·ot·al
/ˈpivətl/
adjective

1. of crucial importance in relation to the development or success of something else.
   "the alliance that played a pivotal role in the revolution"
   synonyms: central, crucial, vital, critical, focal, essential, key, decisive
   "Japan's pivotal role in the world economy"

"Pivotal" appeals to the mechanical, physical sense of a rotational axis, which occupies a single point. Around this point, parts may connect that are otherwise independently mobile, or a rotating body's center of mass may exist. Thus the word implies a sort of unique, decisive importance that isn't value-laden or hyperbolic, but rather neutral and pragmatic.

As such, it might even sound a little unnatural in your intended context:

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

Yet this matter-of-fact sensibility might be useful, especially if you wish to make a clear and practical point without appealing to emotion, as with more positive or alarming alternatives.
It seems that emotional arousal is the problem at hand, after all.

In the context, and digging deeper into what makes one thing more important than the other, consider 'vital'.

Colloquial and short, but I think "top" would do very nicely