Ulic.
His natural grasp of the lightsaber was pretty much as good as it gets, to the point where cut off from the Force, physically past his prime, and almost a decade out of practise, he was able to stalemate a darkside driven and extremely skilled Jedi (to the point where she was capable of slaughtering multiple Massassi Warriors (who were described as being skilled with the dark side) in seconds with her bare hands). In his Force using days where he was in his physical prime, well practised with the lightsaber, and a "master swordsman," it's pretty clear that he would have been a major force to be reckoned with, taking all of the factors into consideration. He did stalemate Exar Kun after all, and while his ribs were broken at that.
As for Force ability, he was capable of blasting two Jedi Knights about 50 meters away from him with one attack, and this before he was branded with Sith tatoos by the Ancient Sith, and growing considerably in power to the point where he was seen to be "rippling" in darkside energies.
I honestly don't see how Vader compares. It was his Force Potential that was described as being around 80% of that of Sidious. In developed power, Force control, and Force knowledge, Sidious still smashes him.