Aspen - I was anti it for the longest time because it just seemed like it would be a PAIN with the amount of urine that comes from adult retics, but after experimenting with my 72" enclosures, the chipped Aspen mixed with some cypress where the hide is, it holds humidity enough, does well for heating, looks good, and absorbs urine great.
I've used newspaper fine, and Rosin paper from homedepot, which is my least favorite. I have used paper towels A LOT and that was my go to, but again, now that I've tried aspen, I use it in all my retic enclosures, but still use cypress in my Boa and Burmese cages.