Forest Floor cypress mulch.
As a random side note I did try to use paper towel for a brief time (exactly one day) in the first few weeks I got my first BP as I thought it might be easier to monitor his health. He absolutely hated the paper towel. He refused to touch it, would only move around the enclosure at the sides, and even climbed onto a ledge on his tub that I didn't even know existed before I found him hanging out there. I took him down from the ledge and tried putting him under a hide.
The next morning I woke up to find him sleeping on top of his hide and that was the end of it. I switched him back to cypress mulch and he's been fine ever since. Won't be using paper towel with him ever again!