You want something that can hold moisture and create a humid microclimate. That means you can choose from coconut coir, cypress mulch, orchid bark, potting soil, top soil, or some mixture of these.
Personally, I now use 100% coconut coir because it retains moisture well, and allows the tortoise to dig. Moreover, it does not harbor fungi or insects the way some substrates can (unless you bake them). It is also quite affordable ... more so if you get it from the garden store instead of the pet store.