Who is the most powerful Sith to have ever lived (EU and Canon)
I am Darth Bane, Dark Lord of the Sith. I will survive. At any cost.
- Darth Bane, Lord of the Sith and creator of the Rule of Two.
The Dark Side is arguably the most powerful side of the force, however, this power came at a cost. A Dark Sider's price for their power could be many things, ranging from an early death to facial deformities. The Dark Side ravages bodies like a plague, it gives power but drains even more.
A good example of this is Darth Bane, he is among the most powerful force wielders to have ever lived. His powers were immense, he could dodge raindrops in downfalls and could create massive force storms. However, by the age of 40, his body began deteriorating at unnatural speeds. He had a constant shake in his hand and by the year of his death, he had been surpassed by his apprentice, Darth Zannah.
This is apart of what is now considered "Legends"