The finger of power!
Rememeber in the movie "Willow" when the little magician, Billy Barty, asks Willow which is the finger of power?
That is kind of how I see this question.
All of the responses about a technique, or practice movement are quite alright, but isn't the most important technique the ablility to continue to learn and grow ... both in physically adeptness and seeing wider context with intellectual learning?
It may not be a technique of pointing to a finger of power on the magicians hand, but realizing that you have the power of Aikido, the power of the universe itself, within you.
It may be a Zen arguemnet that is saying the right answer is the wrong answer, but what do I know? Still ... consider the possibility.
Who is the master ... I am.
Who is the servant ... I am.
What is the technique that is your favorite ... Aikido.
But that is many techniques?
Yep, and a heap of learning to be learned.