No idea on how I stumbled across this.... lol.
Anyways, I am surprised that more people here have not read Darth Plagueis. The book chronicles the series of events about 60 years before TPM through the end of TPM. It is focused on Darth Plagueis and his apprentice, Darth Sidious, their alter-egos, and how they came to realize and fulfill the Granp Plan of the Sith.
One of the biggest points in the book is that NEVER under ANY CIRCUMSTANCES could a Jedi turned to the dark side EVER be as powerful as a true sith. Their anger, confusion, and already developed force sensitivity makes them powerful, no doubt, but a TRUE sith is literally driven by the force and chosen from birth to have a significance to the Sith's goals in the galaxy. Palpatine only had one major fight scene, but the force was stronger in him than in any other being in the history of the galaxy. He was unstoppable.
The Sith and Jedi cycle between phases of power and conquest and exile and defeat... a sequence maintained by the force. Palpatine was the Jedi's reckoning, so to speak. He was the culmination of 2000 years of plotting dating back to The Old Republic... he was meant to be the one to bring the jedi to their knees, and the force made sure he had the power to do so. In turn, Luke was meant to destroy Palpatine. He was his near opposite in force ability and sensitivity, but he did not have the time, place or mentor to unleash his full potential as Sidious did. the force moves in cycles of light and dark. It does not dictate the events of the galaxy, but the will of the force is the endgame of the events and actions in the galaxy, whether the individuals within them realize it or not.
With this in mind, I think that this list really has no true candidates.
Dooku and Anakin were both turned to the Dark Side. They were incredibly powerful, but neither of them could ever have stood a chance against the unbridled power of Sidious.
Ventress had potential, but she was too unpredictable and too rash to be a true sith. Similar to the childish traits displayed by Anakin, Ventress denies her true power by being selfish and closed minded, refusing to see the big picture by letting short-term emotiosn cloud her judgement (albeit fuel her abilities as well).
Grievous was not force sensitive.
Maul: Darth Maul was adopted by Sidious from birth. He was the perfect candidtae to be an incredibly powerful Sith Lord, but Sidious ruined him. Sidious took him as an "apprentice" but groomed him to be little more than a powerful slave. Maul was never given insight to the histories, texts, and ancient teachings of a Sith - the knowledge that seperates a true sith from a force sensitive being fueled by the darkside.
When it comes down to it, I think Maul had the most potential. Had Sidious groomed Maul in the same way that Plageuis had groomed Sidious, who knows what might have happened. Whatever would have happened would have had to happen after ROTS, because that was Sidious' destiny, and Force was ensuring that the Jedi would dissappear (although using Anakin and Order 66 was just hte way Palpatine approached it, not the way the force demanded it).