Paint brushes of all kinds for any paint medium.
Not only will a flat brush create a broad brushstroke, but if you turn it so you're leading with the narrow edge, it'll produce thin brushstrokes.
A round paint brush is the most traditional brush shape, and what most people imagine when they think "art paint brush". A decent round brush will come to a lovely sharp point, enabling you to paint fine lines and detail with it.
A hake brush is an oriental-style wash brush on a long flat handle. It is useful for laying in large areas of water or color, for wetting the surface, and for absorbing excess media.