You will get varying answers on this, depending on who you talk to. There really is no "best." It's a matter of choice.
As I told you via email, generally people use kiln dried pine, aspen, carefresh ultra, or fleece. It just depends on which one works for you. I've used pine for 10 years with no problems. Aspen doesn't absorb odor enough for me, CU is too expensive and they seem to enjoy eating it, and fleece just isn't going to happen here with 180 cages.