It is a fine article. Quite cleary the Katana is one of the worst possible weapons for armoured combat while being an excellent one for unarmoured combat. Kagemusha is right when he outlines that a Yari or Naginata would be a far more likely weapon for the Samurai. A knight could have used against a Samurai a wider set of tools, but IMHO the knightly poleaxe and the late longsword would be good choices with a huge variety of attacking angles and uses. Both were widly used in armoured combat. With a high degree of skill on both sides it would have truly been a interesting fight, ending possibly with wrestling moves which enjoyed high esteem in both martial styles.

BTW: Hardened Plate has a very high chance of deflecting a strong thrust of a narrow point, so it is all about finding the gaps or getting into grappling distance.