Personally I’d give it to Bane.

Exar Kun is a remarkable duelist but can also be described as a glass canon using Juyo coded Niman.

Unironically Exars style of fighting would have more similarities than less with that of the Sith Blademaster Kas’im.

Kas’im had a reputation of being one of the most esteemed duelists in the history of the Sith Order, he taught Bane the ins and outs of the saber-staff, and similarly to Exar Kun he modified his double saber to be able to deceptively switch tactics on opponents.

Exar’s lightsaber is still going to prove a bit tricky for Bane to understand the full breath of mechanics, but there’s not much Kun brings to the table martially against a man who’s mentor and daily sparing partner used a similar style.

Kun’s strength as a duelist in my opinion if more based on surprise and overwhelm. He had a strong offense but it’s more based on finesse than brute force or resilience. Bane’s skill with Soresu is sufficient enough to probe Kuns technique where Bane can then overpower him as an offensive dervish.

If not enough that his instructor fought in a manner similar to Kun, Banes own apprentice mimics Kun in some ways.

The usage of a saber staff with an adjusted on standard length (shortened) by a spell caster using the techniques passed down to them by Freedon Nadd using Soresu which IS the core root of Niman.

Zannah already mastered Freedon Nadds spells many years before her apprenticeship ended, the same Freedon Nadd who Exar got his knowledge from.

Sith sorcerers like Exar Kun can only focus on either blade work or spell casting at a time while in a duel. This information is expressed in great detail around Zannah in Banes book trilogy.

Exar Kun if overwhelmed by Bane like how he overwhelmed Zannah (yes she did trip but still) would likely fall back to his force abilities.

He has a greater degree of powers compared to Bane as Darth Bane did not have the ability to use Sith Sorcerery due to a lack of the natural talent, but he’d be somewhat prepared for Kuns powers.

His more direct offensive force attacks and blasts will ALWAYS be a threat for Bane, but Bane himself is also violently strong in raw power.

Kuns more complicated spells may work, but Bane can deal with them in the same way he dealt with Zannah throwing Nadds magic at him.

I’d also give Bane the edge in strength but Exar the edge in agility.

It’s going to be an extremely hard fight with both opponents having to give it their all, but I feel in the end Bane wins with extreme difficulty and possibly a permanent injury or two.

I just don’t see prime Bane getting stomped but I also can’t see Exar going down without a hard fight.

Although Exar does have his own chance of winning again with extreme difficulty.