Two answers really.....

1. I've always found spreadeagled on a bondage bed (steel frame and plenty of attachment points) among the most practical. Completely inescapable, it leaves you completely helpless and exposed for edging, CBT, NT, and impact play on inner thighs. It is also comfortable enough for extended play where you might be left for periods of isolation and/or sensory deprivation. It's a position that doesn't create any unintentional "bad" pain to distract from the sensations the Domme wishes to impose.

2. The most effective predicament I have been in was standing (well, crouching really ) while wearing a humbler. My wrist cuffs were clipped together behind my back and attached to an overhead line. That line ran through some pulley's to a wall mounted winch.
She tensioned the line to stress the shoulders in a strappedo position. If you tried to straighten up to ease the pain in your shoulders you added tension on your balls. Crouching more to ease your balls only made your arms hurt more.

Simple but diabolical. She gave me a way to get out early but that involved 1 hard stroke with the cane for every minute of my "sentence" I failed to complete.

This was close to 30 years ago but it still stands out in my mind for it's effectiveness and it's intensity.