It’s not about the difficulty of the piece though per say, it’s more about how well you can put you into it and play it well. I’ve had this conversation with many people that have either marked or been through the hsc and have received this feedback.
Stylistic choice, dynamic variation, ensemble communication, and you being the focus are the main points of focus for markers.
Of course pick a piece that demonstrates your skill but allow yourself to learn the 4 (or however many you do) to be amazingly performed. Not amazingly tracked by the original artist and played to a decent level. Feed them flavour. Not sophistication