Since I cannot nominate another Sith, my nomination, following current guidelines, falls to Baylan Skoll. My reasoning is as follows:

He possesses a profound understanding of Jedi ideology and, therefore, mastery of the light side of the Force. Although he did not reach the level of a Council member, his skill with a lightsaber is undeniable. The fact that he currently works as a mercenary alongside a Nightsister suggests to me that he also wields the dark side. This duality, in my opinion, places him above a Jedi Knight or a Sith like Maul, as he understands and can utilize a much broader spectrum of the Force.

Furthermore, like Dooku, he is not only a fighter but also a diplomat and strategist; a combination of qualities that I consider essential for someone aspiring to be Emperor.