Actually I have only recently learnt that most katana would be useless against armor.
The reason is not that they would break or shatter.
The truth lies in the nature of the edge - it was a cutting edge sharpened like a razor. Excelent for cutting through flesh and soft material - even the light armor of samurais.
Since we don't use any true plate armor nowadays, the katana or swords of similar fashion could be the choice of sword.
The problem with a katana against a knight lies in the interaction between blade an armor: Even if the katana managed to pierce the armor, its edge would be probably blunted, flawing the weapon and it would get stuck.
Medieval swords weren't sharepened like that - they were blunt like a buttler's axe, and instead cutting they splinted, shattering the armor.
Knight's swordfight wasn't elegant or graceful - it was force against force where the knight did their best to open the other's full body can with their opener.
It wasn't until rapiers (light swords suited for a time where no whatsoever armor was used) came to be that the modern gracefull sword wielding came to be.
My choice of close combat wepon, the Khopish or Falcata. It was the Macedon's sword and later thrak gladiators used it in the arena.
It is a shortsword, but nowadays I don't think close combat would take place where you have enough room to move a long blade.
It only has a single blade.
Just like the Sica its blade is curced to ward the cutting edge allowing it to reah behing shields, or whatever the opponent tries to protect himself with.
It has a splitting edge just like medieval swords, and although a pure cutting blade may be suited better for current conditions I take no risk - even if I merly crush my opponents bones, I still win.
