Debatable. Old Sith, such as Naga Shadow and Exar Kun, were certainly powerful. Darth Revan was regarded by some as like "looking into the heart of the Force". Darth Sidious is the only Sith that completely wipes out the Jedi Order. Had Darth Vader not been maimed by Obi-Wan Kenobi, it was clear he would have become the most powerful Sith Lord to have existed, but he never managed that since his cyborg body could not handle powers such as Sith Lightning.
Ultimately, the one that caused the most damage and came the closest to conquering the galaxy would have been Darth Sidious, so he's probably the most logical conclusion.