In my opinion the three who could've been the greatst would be either The Sith Emporer (Darth Vitate) or Darth Nihilus.
Now some might say that "according to G-Canon Palpatine is the must powerful sith ever!"
Well my responds to that would be: "Please... Lucas clearly havn't read the expanded univers so in my opinion his words are worth less than nothing, when it doesn't concern something about the movies."
Marka Ragnos and Naga Sadow mgiht also make the list, but since we know little of Ragnos' actual force powers he is too unclear. Sadow might've started two supernovas, but he used a meditation sphere which greatly enhance his powers, for that reason I believe Vitate and Nihilus is more powerful than him.
Comon guys have none of you played TFU? starkiller is by far the most powerful force wielder he was also both jedi and sith
No starkiller is the resolve of allowing incompitent people, who have no regards of lore or bigger understanding of the force, to create a god of war like game.