Gold standard hair: Kolinsky sable — natural taper to a fine point, exceptional snap-back (returns to shape instantly after a stroke), and a "belly" that holds far more water/paint than its diameter suggests
Wash specialist: Squirrel hair — softer and less springy than sable, but even higher liquid capacity; the standard choice for mop brushes covering large areas
Budget/vegan route: Taklon and other synthetics, now good enough that many working watercolourists have switched entirely, particularly for travel and plein air work where losing or damaging an expensive sable brush is a real risk
Shape range: Round dominates watercolour more than any other medium — the point-to-belly range of a single good round can replace several other shapes entirely
Quality test: wet the tip and flick once — a true kolinsky snaps to a single clean point instantly; synthetic blends may take a fraction of a second longer or hold a slightly blunter point