What is the single most important self-defense skill?

Attention. Not a technique.

The fights people survive are the ones they saw coming forty seconds early. The guy who changed direction to match yours. The car that pulled up too slow. The friendly question at a gas pump at eleven at night. Every technique in the world is what you fall back on because you missed that.

I teach it as a habit rather than a lecture. Where are the exits. Who is behind you. Are your hands free or full of phone. What does your body do when you get startled, and can you make it do something useful instead of freeze. All of that is trainable and it is the cheapest thing on the list.

If you want one physical skill to go with it: get up. Not a submission, not a knockout punch. Standing off the ground without turning your back, in a way that lets you move away. Most of what goes badly in a real situation goes badly on the floor. See what to do if you end up on the ground.

After that, in order: keep distance, break grips, move toward people and light. If a weapon is in the picture, all of that matters more and nothing else matters at all. See whether I teach weapon defense.

Grabs deserve naming separately, because that is what it usually looks like. Not a boxing match. A wrist, a sleeve, a bag strap, an arm round the shoulders. Breaking a grip is mechanical, it works regardless of size, and you can learn the shape of it in one session. See what to do if someone grabs your wrist.

For the same priorities with the real situations attached, what a woman should learn first is the short list in order. And why avoiding trouble beats winning is the other half of this answer.

None of it is legal advice about when you may use force. That is between you and a lawyer.