Registered: Aug 2014
Location: The balance of the Force
Zallow's performance against Malgus is honestly more impressive. He not only stomped Adraas with a Force wave prior to fodderising Sith warriors, but he matched Malgus strength-to-strength in augmentation and even scored a hit that dislodged a tooth. He also scales off of Kao Cen Darach, too.
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Within your furnace heart, you burn in your own flame. This is how it feels to be Anakin Skywalker.
Zallow being considered a more noteworthy kill than Darach is a good accolade and Malgus's impression of him in Deceived conveys that he's worthy of distinction, but I think Kit's acknowledgement by three renowned masters in the RotS novel (Mace, Shaak, Obi-Wan) as one of the best swordsman in the order and the nod Obi Wan gives him in the Cestus Deception better substantiate Kit's place as a top dog of his era.
Feats-wise, I can't speak too confidently. I find Zallow briefly contesting with an enraged Malgus to be pretty good. His blitzing of no-name sith warriors (I don't remember any distinction being given to them in the narration) is not bad, but I personally find it comparable to Kit dismantling magnaguards and JK droids, which have their own descriptions attesting to their speed and adaptability in combat. Being acknowledged as a better swordsman than AotC Kenobi while gimping himself in certain areas like speed is nice too.