Brushes are such a personal thing. You can spend a lot of money trying to find just the right watercolor brush. My absolute favorite, hands down, brush is a sable and synthetic blend. the Dreamcatcher series set from cheapjoes.com. I love the that the brushes hold lots of water, have lots of spring and keep their point.
I never use my 100% sable brushes anymore. 
My favorite size is 14 and 8.