Well the best substrate depends on a few things
1. Humidity: if you have a hard time maintaining proper humidity, something like coconut husk or cypress mulch is probably your best option. They hold moisture very well.
2. Cage Heating method: what substrate you use doesn't matter if all you are using to heat your cage is a heat lamp or Radiant Heat Panel, but Under Tank Heating pads require a thin layer of substrate (1/2" thick or less) For Under Tank Heating Pads a paper based substrate is best (newspaper/paper towels/butchers paper/corrugated wrap)
3. How much time you want to spend cleaning the cage: Paper based substrates are quick and easy to keep the cage clean. All you do is remove the old/dirty paper, spray the cage with a safe disinfectant (like chlorhexidine or F10) wipe up the disinfectant, then put in a new sheet(s) of paper. Aspen/Cypress/coconut husk/... Require less frequent but far more involved cleanings. The bigger your cage the harder it gets.
For me I use corrugated wrap. it looks good, it is easy to keep clean, and doesn't effect my cages humidity. My cages stay around 60% humidity without misting so any substrate that holds moisture in will make my humidity levels go through the roof.