Best humid substrate?

I use ZOO MED cypress mulch and so far had no problems with it. It helps me keep the humidity between 55%-65% without the need to constantly spray it. I also have a screen lid, but try to cover 3/4 so that the humidity stays within the enclosure. I cover it with the cardboard cut so that it tightly fits over the lid. Also you can place one half of the water bowl on the UTH so that the water slightly evaporates and increases humidity.

Cypress for the win. I will soon be trying a cypress eco earth mixture soon to see how well that works out. But the cypress retain alot of moisture!

Cypress will probably be the most humid, but I wouldn't count on it being around for a long time. If I'm not mistaken cypress is becoming endangered in some areas.

I really like coco husk