Darth Sidious, who considered himself to be the ultimate Sith Dark Lord.
Darth Sidious formulated the Rule of One as a doctrine of self-affirmation. This replaced the Rule of Two, which was a longstanding Sith doctrine. He thought the Rule of Two was outdated because it had already served its purpose by putting him in control.
The political work Absolute Power by Darth Sidious was the first time the Rule of One was written down. In it, he said that the Sith would be represented by a single Dark Lord, who would be Sidious himself. He would reign eternally as Emperor of the galaxy and would not need to train a successor. Instead, he would only use capable individuals skilled in the Force as dark side operatives to carry out his wishes. While these operatives gained some of Sidious's skills, they never possessed even a small amount of his power.
Before Darth Sidious, Darth Tenebrous, a Dark Lord from Bane's Order, had already speculated that the rise of a "One Sith" was unavoidable. This Dark Lord would be so strong that he would not require a replacement.