I expect that you will get many different responses to this as stuffing is one of those "personal preference" things. It also depends on what effect you want to achieve. On most of my bears, i stuff the heads very firmly using Polyfil by Fairfield. there are other brands which will firm up faster, but I like the control with this brand. I stuff bodies and limbs with a combination of polyfil and plastic pellets for weight and "huggability". I use glass pellets in place of the plastic ones in smaller bears.
Those are my preferences, but there are many, many kinds of stuffing materials so I would recommend playing around to see what you like best.
There is one no no though as far as stuffing materials are concerned, though. Please don't be tempted to use dried beans, rice. corn or any food products as you don't want your bears to attract any tiny uninvited guests
A bit of info I just discovered. I normally buy a 5 pound box of stuffing at Wal Mart. I am not sure of the brand, maybe Mountain Mist. I like this stuffing real well. Last week I was in Wal Mart and they had the stuffing in a 10 pound box. I bought it thinking I will save a dollar or two. Well this stuffing is a different brand. I really don't like it as well. It is slick and harder to get a firm feel. It probably would be wonderful for a softer bear. Fortunately I have a stash of stuffing that I bought a while ago that is too firm. I have found the two make a good match. I just mix stuffings now.
Millie's post is a prime example of what I was saying about personal preference. I buy the 10lb box of Polyfil by Fairfield and the smaller is Mountain mist. Millie prefers Mountain Mist, and I like the polyfil (even for stuffing firmly) So you just need to see what works for you