I use all Holbein Gold Round watercolor brushes. I know most painters use a combination of flats and rounds but I have found that I use all rounds for just about everything I do. Not only do I use these Holbein Gold brushes for everything I do, I suggest them for my students as well because they are more than affordable!
Vermont Art Supply is the best place to find them (go to the store or online purchase) and you can get the link at the bottom of this page.