Chris,
For me, a few months ago, it would have been wire walkers. It took me forever to understand the trick. There are so many factors like tension, top lean at launch, tip shape, string, launch, landing, (I still struggle with landing), string handling for wire walker variants like short circuit and cable car, which tops make it easy to learn the trick, common beginner errors.
For me today, probably orbits, suns and related tricks. Or perhaps various under the leg tricks. I need help on release and top orientation.
Kirk