"Best" is really subjective. The best substrate for you is going to depend a lot on the environment that you live, and what you need it to do. For example, you may live in a super humid area and aspen might work for you, or you might need help boosting humidity, and something like repibark or coconut husk would be better. It really depends on what you need to maintain those parameters for your snake. A little more information about your setup and what your weather is like would help us in making a better suggestion for you.