Revan, easily. According to TSL, Kreia trained Revan as well, and he spent the majority of his time as a padawan, both during and before the Mandalorian wars, researching old Jedi/Sith holocrons, learning EVERYTHING he could about lightsaber techniques and the force, and this trend likely continued when he "turned". After he regained all of his memories, he'd probably be the most skilled Sith around. No way in hell could the Exile take him.