I have a Japanese haké brush made with goat hair. It’s great for holding lots of water and making washes. I have a similar brush to this that is Chinese-made but with pony hair instead. Both are very good for making watercolor washes. I find I have more control with the Japanese haké brush.
I like to use watercolor brushes that are a blend of natural hairs and synthetic bristles. A paintbrush I use is from Daniel Smith, which blends both types of brush materials. The synthetic is essentially plastic. The natural hair is sable from the tail of the Siberian Sable.