I've used allot of different things.
Cork, maple, locust, old pine, poplar, oak, walnut, manzinita (from Andy's).
I have some grapevine, but haven't put anything on it yet.
Actually I haven't been thrilled with the manzinita since it is very smooth, and some roots seem to have problems sticking to it, and it holds no water.
Some of the maple and locust is holding up the best. The oak and walnut barks haven't always stayed attached to the wood well, but the orchids grow fine on them. I haven't had a chance to try it yet, but some people swear by sassafras.