Until quite recently I only used Mountain Mist Fiberloft. It is an American stuffing and quite robust but it does tend to ball, forming little pockets of stiffness that I do not want. So I started to investigate other stuffing.
Poor quality stuffing can ruin your dolls, they don't stuff easily or smoothly and leave your dolls lumpy - so avoid them. One of the worst is Hobbycraft's own, a truly dreadful filling. Having tried several different brands I have settled on using both Mountain Mist with Supersoft Toy Filling by The Craft Factory. It isn't hard to come by and is not too expensive but is very soft and does not ball. I stuff the heads of my dolls with this for smoothness and the body, legs and arms with Mountain Mist for a greater firmness.
You really do need to experiment to find what suits you and your doll style but if I was going to buy only one type of stuffing it would be The Craft Factory one for the best in versatility. Stuff firmly but not over so, this will give your dolls their very best line and show your pattern off to its best.
There are plenty of other stuffing materials available and I know some doll makers, particularly who make for children, talk about natural fillings. There are those who use corn stuffing or wool or even bamboo. Be aware that these are great if you are making a large piece but fall down when making small dolls because again they ball. They are also more difficult to use, I have used wool and bamboo, I like the feel and the weight but not the appearance of the doll so again it comes down to personal choice and the size of your pattern. Finding the right stuffing for your doll is as important as using the right fabric for the body. So happy stuffing!