Substrate?

[DVirginiana]

I use cypress mulch and it works great.

Looks, price, cleanliness, moisture are NOT things I'm asking about

Those are things the snake would benefit from as well. The better a substrate soaks up moisture, the more comfortable your snake will be after going to the bathroom. The better it holds in moisture, the easier it is to keep your snake's humidity where he likes it. You'll never really know what your snake likes or dislikes, so the best thing you can do is keep your husbandry top notch and that'll keep your snake as close to 'happy' as they ever really get