A watercolorist uses two tests to define how good their brush is - will the hair hold buckets of water, and are the points sharp. Natural sable is best. The bulbous hairs found in nature snap to a sharp wet point that can cut glass and reservoir water naturally. Every watercolor artist likes a brush that can lay down a copious wash in a single stroke or provide the most delicate point for details like twigs and whiskers.
Synthetic watercolor brushes can still be quite good and will also keep your wallet a little heavier.