Here's a tough one...

[Touch Of Death]

I would disagree. Thinking of nothing is all well and good but it must be a disciplined nothing. Not only that, when under attack you should at the very least be thinking positively, as in, "I was hoping you would do that!"
Sean

 

[OC Kid]

I was going to say any technique you master. The masters of old knew a lot of techniques and mastered only one. One is all you need, No matter what the individual technique is, reverse body punch, shotei what ever. If you master it you can use it in any or all situations...JMO

 

[Marginal]

Marginal,
mushin is a part of it, but just like all the previous answers. Relearning innocence? beats me...

Just a reference to an old Japanese text by Yagyu Muenori that at one point advocates training until whatever techniques you're integrating are completely natural when they're executed as if they're instinctive etc.

The answer is "nothing". there is no one technique
no technique, but at the same time it is the technique at hand...

Technique's still there however. If nothing settled it alone, then nobody would even need to bother with a MA.

Like when your hand shoots out for your grandparents wedding set that you just nudged off the table. The moment where there is nothing in your head, that split second, just before you start thinking, OH, SH##!!!

Usually in those moments for me, time seems to slow down. Not really a no thought situation, just one instant of obvious, imperative action.

For example, back when I was four I was coming out of my room, and I saw my 6~7month old sister crawling off the back of our couch. Resulting in her sailing headfirst towards the floor from about 4' up. I sprinted down the hall, and caught her about 6" from the ground. Crossing that gap took almost no time, but I saw every millisecond pass like a minute.

 

[Shotochem]

IMO, the most powerful technique is any technique taht is delivered with focused INTENT to do physical harm. There is a big difference between your typical punch and one coming at you with the intent of really hurting you.

 

[MisterMike]

Powerful is a relative term. My answer is "No Technique" as well. This shows you have true understanding of Karate. It is in line with Musashi's way of "No Sword".

With this understanding, and I am of course still learning, you can beat an opponent without throwing that punch. Or you can use your opponent's punch against him.

Think back to Mr. Miyagi, avoiding the punches at the beginning of the second movie, and Mr. Cobra Kai guy cuts his hands all up. He was defeated physically as well as mentally.

Having a calm mind in the face of having to accept whatever is presented to you.

It seems your teacher's question is more of a Koan. It get's the student to look deeper at the inner layers of their art and then at themselves. To me, this is the ultimate goal of Martial Arts.