Well, Starkiller's the strongest, no doubt.
Lumiya's second.
Followed by Flint (who was a tough fight to RotJ era Luke), then Grand Inquisitor Tremayne (who was the one that recruited Jerec to the dark side).
Most of the rest were adepts and inquisitors, who'd be significantly outmatched by any of the above. Even though there's a fair number of people who've been taught by Vader, there's apprentices and there's apprentices, you know?
Maybe if Ashoka lives she'll be up there ^^
A lot of the stronger force users in the Empire weren't trained by either of the Sith, probably purposefully.
They let Cronal (of the Prophets of the Darkside, who learned under Palps until Palps tasked him to enslave the spirit of a sith king and learn from that. Palps did have a few good secondary students. Vergere being another, and Mara Jade) teach Jerec and then Jerec teach several more Dark Jedi. Or the likes of Carnor Jax, who despite being strong in the force never got teaching in it until Lumiya trained him.
So a lot of the best candidates were kept two or more steps away from the Sith.