Ishin probs takes it, but a lot of people would sleep on Merc; his regen factor even tho limited to an hour makes him practically immortal. Ishin's Black Fire (the strongest ability in the fight) doesn't do anything to him as intense heat makes his regen stronger and he is resistant to magic. Although Merc is vulnerable to beam/plasma which Ten has a plasma sword.
Ten wouldn't take this although his standard equipment has many advantages over the other combatants. Most importantly, he is too much of a showoff and lacks the strategic knowledge of Charlemagne and would overwhelm himself attempting daring maneuvers that ultimately get him killed. Akira would suffer the same fate for a similar reason.
Morte doesn't have the powerset to go the distance but would be dangerous early on.
Charlemagne's Chi/Ki based combat would carry him until the end and possibly end with fighting Ishin.
But I think Ishin, even after dealing with all the swordsman on the list eventually wins. Or, everyone dies from mortal wounds.