I prefer cotton or a cotton blend. But it does depend on what you will use the amigurumi for. If its for kids, use safety ears and look for a yarn that can be washed on 60 C / 140 F. If its for yourself, choose a yarn that you like and dont worry.
a lot of people recommend cotton but i cant afford much so i use cheap acrylic, currectly using whats left of Caron Simply Soft but ill use store brand yarn since its around $4