AncientPower
The Chosen One
Because Ulic Qel-Droma shitstomped Warb Null on a dark side nexus before becoming 'immensely' more powerful, Warb Null was stated to have been an excellent swordsman before he'd even become Warb Null. Warb Null then become much more powerful, was stated to have mastered the dark side and had gained the combative memories of Freedon Nadd and King Adas from the armor itself.
Fast forward a few years and Ulic has become 'immensely' more powerful, with powers 'far greater' than any of the Krath, and was already one of the most powerful Sith ever. Ulic is a Luke Skywalker archetype, he can use great powers externally but he is mainly an internal powerhouse who shows it through his Force augmentation.
When fighting Kun he is enraged by Sith poison and is obviously further enraged by Kun's attack on Aleema Keto. So this was an absolutely all-out Ulic who in a far less powerful state can stomp guys like Warb Null.
Then Exar Kun doesn't just get more powerful after this fact, he gets ancient Sith combat techniques from the most powerful Sith holocron in the mythos, the Dark Holocron. This is where he obtains designs for his unique saberstaff variant and ancient Sith fighting styles. The very same teachings that Kreia states renders the modern era duelists infantile in their skills.
As far as Vodo Siosk-Baas goes, he is stated to have never stood a chance against Exar Kun's 'deep reserves' of power and his saberstaff. More importantly, I struggle to think how exactly Vodo is incapable of countering Kun's saberstaff, when he uses the weapon in the same fashion as a quarter staff, the very weapon Vodo Siosk-Baas not only uses but is a Jedi Weapon Master for his mastery over it.
Kun is certainly the greatest, given he was capable of stalemating a Sith as powerful as Ulic Qel-Droma for hours before growing much more powerful at a fast rate after gaining the Dark Holocron and then completely reworking his fighting style so as to adopt ancient Sith combat techniques from the single greatest source of Sith information in the mythos, makes him pretty much one of the most all around solid duelists ever.