There is no best, it depends on what works for your animal, and spending on what you do or where you live it might be a very different substrate.
Aspen is fine but will mold fast if soiled, it also does not help with humidity, ow if you leave in the south that is not an issue if you live in a dry area it will be.
Coconut bedding is fine and keep humidity.
Newspaper works great easy to clean and if breeding prevent any pieces of substrate to get in the wrong area.
etc
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