Very often the one that comes with the mouthpiece. Some people think that the ligature can affect the sound. I’ve never noticed this if the ligature is working properly, though one of the flexible type ligatures will sound better than a distorted or ill fitting solid metal type. If the table is concave (or heaven forbid) convex, or if the reed is warped, then some ligatures will work better than others with those specific parameters, and may therefore give you the impression that ligature A is generally better than ligature B, or that each ligature has its own sound