Simply put, Kun is canonically below Malak, while Malak is canonically below Corran Horn, and prime Krayt should be above Corran Horn given he was demonstrating the ability to slightly approach Luke Skywalker in FOTJ. Meanwhile Corran Horn is inferior to Kyp Durron by authorial intent, and Kyp Durron sucks.
Exar Kun is better than Karness Muur, who cheapshotted terminally ill backstabbed Darth Krayt, therefore I agree that Krayt has a better chance of winning than Exar Kun, because nobody is here to stab Krayt and Exar Kun was defeated long before Krayt even reached his prime anyway, meaning they would never meet but Krayt would still be alive
Because the Force fights through an individual as an expression of its own will, that individual as a vessel for the Force has to varying extents the capacity to be worthy of being filled with that energy, its on a spectrum, and thus their state of mind becomes an important factor. If they don't believe in themselves, they cannot channel the Force, that is why emotions are important and those people that can fight impossible odds and still believe in themselves (see TPM Kenobi @ Phantom Menace or Hero of Tython fighting Darth Malgus in Star Wars: The Old Republic Massively Multiplayer Online Game) end up making the impossible possible, its like how nobody could run a 4 minute mile until someone did it, and then everyone could run a 4 minute mile and even some people are doing it in 2 minutes now.
So the overarching and main point is that battles between Force users are like a game of philosphy, but also a game of dejarik, if you doubt yourself, and you don't believe in the cards you have been dealt, you might as well fold because its checkmate, and while Malak folds to easy odds (weakened nexus hurt Revan who fight literal armies of robo-droids and force wielding assassins) Corran Horn fights against literally impossible enemies and wins (Shado Shai, who is an invisible-to-Force Vong user who he cannot even sense and has a poison snake that can tangle lightsabers).
I think in the end this would be like Kyle Katarn in video games versus big Force users like Jerec and Desann, on the surface level he ought to lose, even if he brings a blaster and thermal detonation packs, because he is just too weak, but because he believe in himself and the Sith are able to fold under possible odds, Kyle finds a way and collapses a pillar on Desann head. In the same light I believe Corran would be able to make Malak be crushed under some massive weight like a pillar, or maybe even under his own self doubt and loser mentality.
If you come out the winner, and you don't lose, but the other guy is in the end dead, who is actually more powerful? Because if your actions and beliefs lead you to the kind of situations where you are dead, and cannot be an actor in the universe any longer, then no matter how "conventionally powerful" you are, you cannot wield any power from the grave. Then there is Karness Muur who lurks in the limbo world of the undead, snatching any chance he can to defeat his enemies, you can say Krayt is more powerful than him, but at the end of the day, Krayt flew off a cliff like a tardburger and Muur stood the test of time, in that sense, in a very real way, Muur is more powerful due to not ending up as a corpse.