The one that I need the most improvement on. I can always do a technique I am comfortable with. But if I need to pratice on one then when it is time to use that technique in real life and I can not do it cleanly then that is the most important technique there is and I do not want it to fail me. We have to remember that uke/attacker will dictate the technique we use in response to the type of attack and the way uke has moved. I need all of my techniques to be as perfect as I can make them so that they never fail me.
Principle wise, I would say blending is the most important thing because it is one of the hardest things to get good at so that the techniques become effortless.