What about Maul?
If SOD Maul was roughly around Dooku's level(I don't think that's debatable) after losing half of his body(and thus, Force potential) and spending a decade as a junkyard dog, a Maul with his full body and a decade or two more of apprenticeship under Sidious would be incredibly powerful.
Vitiate - considering all of his cosmical knowledge on the dark side, all the rituals he made to become more powerful, etc. he should in theory shit on Sidious [he doesn't] and be waaay more powerful than he actually is.
Kyp Durron - what NewGuy said.
Meetra Surik - She was all good and powerful until she got shit on by Nyriss. I give her more credit than most of people here, due to the specific circumstances of their encounter, but it was still a disappointing screw up [yet another one] from Karpyshyn.
Jaden Korr - wasn't he supposed to surpass Kyle by a considerable margin and become a really powerful Jedi?
Vader - I'm not sure if his stagnation in relation to his potential has to do with his limb losing and 'more machine than man', or if it has more to do with his psychological and mental states, tbh.