Depends HEAVILY on the situation.

For my tubs the snakes could be in there on nothing and the humidity and temperatures would be right. So I like packing paper (unprinted news paper) or aspen. These tend to be dry substrates, difficult to maintaun humidity with.

For my lizards I give them a mix of sand, eco earth and dirt. Also could be using moss in the mix, but I didn't have any on hand so I didn't bother. Never changed their substrate.

For my rats I like to use eco earth.