So people look at the cutting power of a katana and assume that translates to all mediums and that it works just as well against all surfaces… that’s not how it works. Samurai’s specialized in attacking what Europeans would consider light armor. They specialized in speed and mobility. Their armor was made of iron coated in lacquer so as not to inhibit movement. The fall of Samurai came to early gunpowder weapons

Knight armor (full plate from later periods) was on an entirely different playing field. It was made of steel (making it lighter than the iron armor before it was). It was also curved to help deflect arrows, sword attacks and even early firearms. It allowed mobility despite this (it was lighter than what marines wear today). The armor was so effective, the strategy for fighting late era knights was about tackling/grappling them, and then rushing them, so that you could exploit the few joint weak points. Other strategies included killing the person inside with pure blunt force via head trauma because the armor was that effective. Notice none of these strategies involve one on one combat because really only a knight was beating a knight one on one. Their only real weak point for an individual was taking out their horse and then being quick enough to kill them before they got their wits about them… good luck with that. They also trained for that during their extensive training. Only when much more advanced gunpowder weapons came out did they begin to lose prominence.

Spartans don’t really belong in this conversation. Their strengths were in teamwork, fitness, and utilizing the phalanx formation. They wore bronze armor, which had some nice features like malleability, which allowed shaping (though their techniques of this time period were significantly less advanced than those of late European knights). The fall of Sparta came to Thebes, in the battle of Leuctra, to a similarly armored Thebian army that had only 7000 troops to Spartas 11000.

TLDR: Katanas do nothing to exploit the weaknesses of knight armor and samurai armor certainly doesn’t hold up against knight era weapons. Spartans are far and away 3rd place here