As for natural potengial Anakin Skywalker most likely has the most, although Vader mused his son had greater potential, and Sidious mused Leia had even more than that. The latter two are character opinions, though, so take them with a grain of salt. And I guess there's Kyp Durron and Galen Marek, too, with supposedly both having Luke-level potential.
For artificial potential, there's probably Sidious and Plagueis if they had perfected the art of midi-chlorian manipulation. And I guess Vitiate, if the galaxy-eating ritual had succeeded, but the guy fails at everything, so it's no surprise it didn't.
Arguably the second-most-powerful Force-sensitive under the rule of Darth Sidious.
[Jerec's] embrace of the Force's dark side gives him powers that rival Vader's.–Jedi Knight: Dark Forces II
Since we know Vader was "the greatest treasure of the Sith," Emperor Palpatine's "minor masterpiece."
He believed he could eventually overthrow the Emperor.
I do not mean to be the Emperor's servant forever.–Jerec (Dark Forces: Soldier for the Empire)
Palpatine himself acknowledged that he was a threat.
During the Galactic Civil War, Jerec was given access to Emperor Palpatine's Dark Side Compendium, and believed that he could establish the Dark Side Elite on his own. Palpatine sensed Jerec's intentions and cut off his access to the tome–The Complete Star Wars Encyclopedia
Inspired by glimpses into the Emperor's Dark Side Compendium, Jerec intended to bring to fruition an experiment Palpatine had spoken of in his malignant tome: the creation of a unit of seven Dark Jedi perfectly absorbed into the Emperor's will, a "Dark Side Elite." Unfortunately for Jerec, the Emperor readily recognized the darksider's ambition and denied Jerec further access to the Compendium, ending the audacious experiment. Nonetheless, Jerec at least managed to assemble a cabal of Dark Jedi willing to do his bidding.