14THE GRAND INQUISITOR
While the Grand Inquisitor isn't a true Sith, he is still skilled in his command of the dark side of the Force. Hand selected by Darth Vader as the leader of the Inquisitors, the Grand Inquisitor demonstrates his skill with a lightsaber in repeated confrontations with Kanan Jarrus and Ezra
Bridger. While we don't see it, the Inquisitors were chosen to hunt down Order 66 survivors and Force-sensitive children.
Since the Grand Inquisitor is both feared and respected by his fellow Imperials and by Kanan, it's safe to assume that he was fairly successful in the run-up to his final encounter with Kanan. His unique spinning double bladed lightsaber certainly gives Ezra and Kanan trouble in their confrontations. Eventually, it's not even his skill with a lightsaber that is defeated, but rather Kanan exploiting a seemingly very obvious weakness in his lightsaber's design.