i like aspen. it holds moisture and small enough to do spot cleaning (where i don't have to remove big chips of wood). it also allows my snakes to burrow and stuff.
i would use white/blank paper for a sick or quarantined animal so that it is easier to monitor their poop, pee, parasite, etc. paper is also easier to clean tho - just crumple it up and throw it away. many folks prefer this.
I'm seeing a lot of suggestions for aspen in in this thread. BUT, just so it's said, if you have trouble maintaining humidity you should consider a more moist substrate like EcoEarth/cocohusk or cypress. Aspen is good but also quite dry. It will not have the same positive effect on humidity as these other substrates. (Especially if you plan on misting your cage a lot)