In what? Combat ability?
I'd put several people over him. Luke, Palpatine, Yoda, and Bane for sure, and Mace, Kyp Durron, and Exar Kun are distinct possibilities, the latter two only on account of their prodigious ability with the Force.
However, certain ancient Sith should definitely be looked at. Freedon Nadd, for one, was said to have learned under the then seven-hundred year-old Naga Sadow for a time before presumably killing him - so his knowledge base and experience would outstrip Exar Kun's one year with Sadow's notes.
"Freedon Nadd had been a Jedi who turned to the dark side as the apprentice of Naga Sadow, the former ruler of the ancient Sith Empire. Sadow's power had been so great, it had allowed him to survive for six centuries, fueled by the energies of the dark side. As his apprentice, Nadd had absorbed all his knowledge and teachings, transferring them into the Holocron before murdering Sadow and taking his place."
Darth Bane also noted that despite having the formidable build and stature of a waaior, his body looked like it had been ravaged by the power that he wielded.
"Though Nadd had been human, his avatar was the image of a man who had succumbed to the physical corruption that sometimes affected those who delved too deeply into the power of the dark side. His skin was pallid, the flesh withered and sunken, and his eyes were glowing yellow orbs devoid of iris or pupil. Despite this, he still appeared as a formidable warrior: broad-shouldered, clad in heavy battle armor and the helm that had doubled as his crown when he had proclaimed himself king over the nearby world of Onderon."
Even in death, apparently, the dark side clung to his remains and formed a sort of point of concentration around which things simply died.
"Even the plants and trees bordering the clearing were stunted and deformed, corrupted by the dark side power that clung to the remains of the great Sith Master in death."
So really, I think we should be as impressed with Nadd as we are with Kun, if not moreso, although that's a topic for another time. Naga Sadow himself is also a person of interest here, and appears to be quite powerful himself.