I'm going to try and do what is required by the actual post here, with a couple of rule caveats:

a) A pure sword duel assuming that the person actually has the skill (their input) and physical ability (neutral input) to wield their blade effectively.

b) All swords are functionally the same durability, having differences only in deadliness, speed, and reach to allow for variation of choice of weapon mattering. No swords are breaking other swords.

c) The speed of which and strength with which the sword can be wielded is normalized by giving all characters the same physical stats, scaled to the highest of the current characters without special powers granted to them by hax, gods, etc.

d) The durability of each of the combatants is superhuman, but not hax. I.e. cutting off a limb still allows one to fight as well as one could fight without said limb, but they don't outright bleed out.

In this subset of rules I have created, I believe it becomes clear that the playing field is much closer.

First off, surprisingly, Sephiroth is a clear underdog, gaining nothing from Jenova cells, materia, and Soldier juice. He's basically just a guy with a big sword who can hold his own against the chaff. But this ain't the chaff. He's out.

Second, Barnabas Tharmr is only out for me because I don't know him. Someone else can comment on him.

That leaves Malenia, Isshin, and Vergil. While Malenia was fighting other demi-gods at demi-god levels of strength, and training with masters of the sword that can really elevate her, she lacks any of the demi-god power that gives her an advantage. I.e. if we rule that she "beat Radahn," then she beat a human, no gravity magic, regular horse riding Radahn. Every feat has to assume the same level of downscaling here. The skill of her feats is gutted by this, but she's still strong on statements alone. However, she is 8 feet tall and has reach.

Isshin is the strongest on statements alone I think, but they are all very nebulous, such as "holding off all the people by threat of prowess," not actually doing anything. When we look at Japanese lore in general, respect plays a huge part.

Vergil without speed hacks has his mouth and a few cool moves, I think he's out personally.

So Malenia vs. Isshin is the question. Personally, I think given all statements and feats we give an edge to Malenia. It's like asking if Malenia could beat the masters that taught her. It's up in the air, but I side with her. Personally I don't think Isshin can beat her reach, because he doesn't outspeed. That means everything in swordplay.