I prefer cotton or a cotton blend. But it does depend on what you will use the amigurumi for. If it's for kids, use safety ears and look for a yarn that can be washed on 60 C / 140 F. If it's for yourself, choose a yarn that you like and don't worry.
a lot of people recommend cotton but i can't afford much so i use cheap acrylic, currectly using what's left of Caron Simply Soft but i'll use store brand yarn since it's around $4