I use cypress mulch and it works really well. I have about 6 inches of it in the enclosure with some hills and valleys. It holds moisture pretty well, but I also added a bunch of spaghnum moss mixed in with for even better moisture retention.
there isn't a "best substrate" really....there are a couple of better options and then it's just personal preference after that.
for a hermann you could use regular dirt/top soil. from your backyard or local lowes. if you purchase it, make sure it doesn't have anything added to it like fertilizer or those little white balls. if you dig it up you should either freeze it or bake it to kill any bugs/eggs and make sure there's no glass in it (it's happened to me!)
you could use mulch as well, nothing dyed. i suggest orchard bark. i good idea is to mix mulch and top soil.
I use a mixture of soil and coco coir (about 60-40 split) with some sphagnum moss in a couple of humid hiding spots. It's good for digging in, but I second the warning about checking the soil first. I got some "organic" stuff for Ringo from a garden centre, and I've lost count of the number of pieces of glass and plastic I've foound in it.
I also use cypress mulch. I found that coco fiber and dirt get in my Herman's eyes easily and bother him. Cypress mulch retains moisture pretty well, and let's my little dude easily dig down