I have recently started using Repti Bark and I like it a lot. Especially with the brighter snakes. It really shows them off! Most people will probably vote for Aspen, with plenty of good reason, it's highly absorbent, stays dry, meshes together to form tunnels as the snakes burrow through it and it's economical compared to some of the others. I have used Care Fresh , Aspen and Repti bark all without problems. Just remember not to feed on the substrate