Well I can't decide. Is Vadar more powerful than Tyranus? Is Cidious stronger than the both of them? And what about Yoda? And even though he got his ass kicked by Tyranus in Episode II, he held his own against Vader in Episode 4.
Also, Vader never did the lightning thing. Tyranus and Cidious could both do it.
Well, why call him that! When you become a powerful dark lord, you are referred to as "Darth", a title of respect. He will always be ither Darth Cidious or the Emperor to me.