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Using a Wacom tablet with:

* A drawing app e.g., OneNote or Sketchbook or Krita

and

* A screen sharing app e.g., join.me

Has worked well for me. What's particularly nice is that you have a saveable, shareable, modifiable artifact when you're done with the remote session.

I'm also a bit surprised that Wiimote whiteboards never really took off: https://youtu.be/5s5EvhHy7eQ

Also check out Astropad https://www.astropad.com which turns the iPad into a Wacom like graphics tablet.

Couple that with a drawing app and it works great

I can only recommend groupboard, not sure they have an iOS app but their web app is as close to whiteboarding as possible: only focus on getting your idea on the board and not about styling and other fluff

To add to the list, Skype for Business has a whiteboarding option. Easy and free.

Pixelboard for iPad