Fleece is the best, because the threads wont fray (which they will chew on) like any other fabric will. It's also soft and absorbent. I still think fleece is the best...Easy to clean, and completely chinchilla safe!
Fleece is the only one. It doesn't fray threads that the chin can ingest and cause impaction like other fabrics. That said, if they do chew extensively on it you should remove it altogether as it still poses a risk if consumed.