I use cypress in my tubs, and like others have said, the snakes seem to like it and it holds humidity very well. Doesn't mold and it really isn't that messy most of the time. If I'm short on cypress bedding, I will use paper towels and these seem to work pretty well too for me, BUT I find that whenever a snake does a poop or pee, I have to change the whole tub and can't just spot clean. It's great to absorb the liquid, but then it's all over the tub and smelly.
In my display cages (which I'm trying to get rid of) I use Eco-Earth. I LOVE this stuff and I think it holds humidity even better than the cypress, but when the snakes strike and miss the rat, they get a big mouthful of the stuff and they really hate it and struggle to spit it out for a minute. For some reason though, in my tubs, it never seems to dry out. In fact, it just seems to get more and more wet from the water dish spilling until it's sopping wet.
Still trying to work out the kinks here and there!