I would pick single whip or dan bian.
It is used as a connection, turn around, start and end posture dispersed out in any tai chi routines or forms.
Why ?
Both our hands start in front of our center line and then drawn out fully or partially to the left and to the right.
We sit on hip and lower our posture or center our posture, zhong ding shi.
What is yours?
and why ?