I legitimately don't know. His showing against 19 ABY Luke would easily place him as superior to the likes even of Dooku or Exar Kun's level, but it just doesn't add up to his other feats, to his origins, potential, speed growth and training. It's just that Luke in that timeline is a character the writers clearly portrayed very differently; you have his power and demonstrated ability jumping from level to level constantly, defeating threats that would be beyond capabilities of Windu-level guys and then having legitimate trouble with Droidekas. 19 ABY Luke, considering Dark Empire and other high showings, should be legitimately so insanely great that Kueller's feat would instantly grant him Dooku level, but then again, Kueller's showing against Luke comes right of nowhere; it's as if you suddenly had Ki-Adi Mundi besting Maul. I really contribute Kueller vs. Luke and their overall demonstration to the fact that Luke's depiction in the EU is by that time straight clusterf*uck.
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They are angry because they have been forced to recognize that their hour has arrived; that the time has come to surrender power to Shimrra and the new order."
Kueller is superior to Brakiss who can cause solar flares and implied he can cause a star to go supernova. So Sadow+
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
Kueller's contention with Luke was due to a rage-amp and Luke being barely able to stand on one leg, and Luke still gave him a good fight.
Also, it's been confirmed that Luke usually holds back against his former pupils(look at Brakiss as a major example; at one point he's a "match for Luke" then later Luke proceeds to stomp him in two blows when he goes all-out).
I'm not quite sure where to rank Kueller, honestly.