Master of Darth Sidious. He learned how to gain immortality through the force.

I don't think he was the most powerful, but I believe he was up there

I believe that he does have a role in how vader was born, considering he could manipulate midiclorians in the ways he saw fit

He most certainly was not the most powerful sith in terms of combat. His skills were in politics, science, and other disciplines. He was the one who set up Darth Sidious for success politically. There is a little issue where he may be indirectly responsible for the conception of Anakin due to his "playing god" with midichlorians.