Learn English – 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.