I use flannel on the outside and corduroy on the inside but that is just me. I have some big diggers and have had a few throw up fleece that they have ingested it after digging and then cleaning the fuzz out of their feet. It really depends if you have diggers (and subsequently feet lickers) or not. Most are okay with fleece, and most people have no issues with it, I have horrible luck and have found that some of my hedgies need something more durable. No threads as Kalandra says is definitely the most important thing.