for Kyusho beginners is what is the most important point to learn?
Well that is a simple question with no simple answer, not all points are the same, nor is the physiology in all people.
Going deeper, not all points are the same nor are they workable in all situations and this must be realized and trained accordingly. It will all depend on the situation, the individual, the attacker and of course your skill level.
Let's take the branch of a nerve most think is the Radial Nerve, located at the inner part of the forarm, just in front of the elbow crease. I did not call this L-5 as most because it is not a single point location, nor is it multiple points named the same. The target is actually the on the Lateral Antebrachial Cutaneous Nerve (this is why however, we use the point names as the simplicity factor is there, just do not confuse it with the acupunture point... it is larger than that).
Typically this point is demonstrated and has some dynamic and impressive results... plus pain and dysfunction, always entertaining. Then after the practitioner can apply it in an exploratory and voluntary experimental application of pressure or striking, the next step is in mock attack scenarios such as punching or grabbing defenses. This is where the potential hazard in training takes place.
The practitioner will then train on these mock techniques under the impression that it will have the same affect in a real attack, this is where the potential hazard becomes ingrained. The potential hazard is that the impression they saw in applied experiment will not work under real intentful attack and they will be caught off guard.
To explain what I mean, this target must be fully understood (see "What's Important" posting). This nerve is laying in between and under muscle, fascia and skin that when a clenched fist is made, constricts and protects (seals) access to the nerve. The harder the fist is squeezed the tigher and denser the muscle becomes and further protection. So if a person throws a punch with real intent, the fist will be very tight as will the protecting muscles. This is also true when they grabbing type attack, but even more so as when an attacker grabs the victim, the also tend to pull the victim in closer for added strength and control of them. This constricts the muscle even further and also closes the elbow to hinder accessibility and penetration to the nerve further.
It can be use if you can access it with the correct penetrating force in some situations like in joint control manuvers, but must have the hand open or have accessed the point before the opponent has the opportuinity to clench the fist, tightening the muscles to hinder your penetration depth and accessibility. But here is a question for you and only you can answer for yourself; if a point is only usable (realistically), 50% or less of the time, is it worthy of training for you? Or would you be better off concentrating on another target proximal that is far more available, accessible and will not be protected with simple and reflexive actions like closing the fist?
You need to not only be aware of these physical restrictions, you need to train properly in accordance with the physiology of real attack. Your Instructor needs to know this not only for this target, but for all targets they are teaching. If they have not mentioned this, you should ask about it as they may not understand it and should for all future students.