I use coconut husk.
I've tried aspen, bark, fir chip, and paper. I didn't like any of these.

All wood based have a chance for impacting, and getting stuck in mouth or teeth.
I found paper wasn't very absorbing and if not caught in time they would smear ...

I also use fully enclosed hides with the access holes on top and carpet inside. I feed in these so no matter what substrate I have, there's no chance for ingestion while feeding.