I don't know the best but I use shredded Newspaper indoors, straw outdoors. I would use straw indoors but I do not want to bring in pests from a farmers wheat field into my house. Pine Shavings seems to be what all the feed stores use when they sell chicks so maybe that's best. I personally like shredded newspaper because the top layer seems to get stuck together like a sheet of paper glued by chick manure and I can just peel a layer and add more swiftly to clean it up.