I've used:

Aspen
Cypress Mulch
Eco-Earth
Paper towels
Butchers paper
Newspaper
& Corrugated wrap

With my BP's all have their advantages and disadvantages.

Cypress and eco earth are great for boosting humidity, Aspen looks good and smells good, but all three of these can be a mess and a pain to deal with in larger cages (in my opinion) Personally I prefer paper as it provides the cleanest/most sterile environment for my BP's. If I wasn't using paper based substrates I would use eco-earth.