The absolute best brush to use with paints is a flat, round-tipped lip brush. Paints have a similar density to lipstick, so you want something that's suited to moving heavier product around without it getting absorbed by the bristles.
The absolute best thing overall, however, is your finger.
The heat from the pad of your finger makes the product more pliable, which in turns means you can apply it smoothly and in a thinner layer.