Forest Floor cypress for me. Thin layer.
Is great for humidity for glass tanks, if you have those, but can push the humidity way up in my PVC Cages (especially when my house's ambient humidity is usually 55-60%). So I just leave the bag open when stored or spread it out in a bin prior to cleaning the cage so it can dry out. Works perfectly in my house if it's just a bit drier than it comes in the bag.
For me, it has also never stuck to stuff the way more "mulchy" substrate seems to, so I never am picking it off hides or water bowls like people complain happens with Eco Earth. As long as my rats are dry, never really had a worrisome case of it sticking to those either.