Eco Earth Coconut Fiber. It really helps with humidity. Though when It's not damp it gets kinda dusty. It's a pain to expand and dry out but It's $5 a brick and lasts a while. If I had more money to spare on substrate I'd get Repti-chip or Reptile Prime.
Mine are currently on eco earth mixed with some cypress. I like the look of it and it helps with humidity a lot. I like being able to spot clean and use a small flat shovel to clean up after them. Rehydrating can be a pain if doing a lot, I usually mix 1 block with 12-16 cups of boiling water, let it sit for a day or two and spread it thinly over the cage to replenish from cleanings. I had them on paper for awhile, cleaning and disinfecting the cage is way easier, doesn't get into tracks if you have sliders, doesn't get stuck to prey while they eat. If your fine with the sterile look then paper is awesome, the look is the only real advantage of other substrates to me.
I use #40 lb butcher paper cheap an easy to clean. I found Cypress mulch great with humidity but just too costly an Aspen was cheaper but way to dust for pythons an boas not mention it molds easily.