The Swordmaster Trope
So here's the common trope: there is a master swordsman. This swordsman has a single Sword of Power(TM) and has never been defeated in battle. There can be only one, etc etc. The swordsman will face many opponents with many different kinds of weapons, and emerge victorious every time. Duncan McCloud from Highlander. Too many examples from anime and manga to count. Jaime Lannister from A Game of Thrones, apparently. Drizzt Do'Urden. Zorro. They just pop up everywhere, when you think about it. The idea of the swordmaster is a very powerful, romantic one.
The problem is that one swordmaster does not an army make, and although it serves for good storytelling SOMETIMES, it doesn't make for good tactics.
One Sword Against Many
Part of the trope that's most problematic is the idea that one highly skilled individual could hold off an army. And... nope. Can't be done. One single person could take several others in a fight, if they had the right weapon and skills, but the idea of one sword against many is, well, fantasy. It requires magic, or high tech, or [insert whatever special sauce you like here]. But skill alone, in a world governed by normal physics, means that one skilled individual will become a speedbump, absent very particular conditions. "Very Particular Conditions" include stuff like intimidation, guerilla tactics, shock and awe, a confined space - there are instances of a single soldier tearing up opposing forces of dozens or even over a hundred others by surprise and terror alone. But those instances are rare, and I don't know of any that involve swords alone. (This is not to say that it never happened, just that I haven't been able to find any examples.)
There are physics in play with swordfighting that dictate just what you can do with your big steel lever. Even a swordmaster with a superior sword will not be able to move the blade fast enough to defeat, say, ten guys with spears and far better reach. They might kill one or two, yes, but after that they're getting pinned to the ground.
The Ultimate Sword
Hahahahahahaha No. There is no such thing. Short of adding some special secret sauce, there is no such thing as the bestest sword ever. If there was, then the design would have taken over everywhere, and obviously that isn't the case when we look at history. Swords are incredibly varied, and they have different things that they're good at. A katana or sabre is great at cutting, but a rapier is far superior for thrusting. One swordmaster against another with a different weapon is all well and good, and it makes for interesting fights, but the situation doesn't always allow for fairness! A longsword versus a rapier on the battlefield favors the longsword, whereas the same in a formal duel favours the rapier, solely because the weapons were designed to be used in different situations.
Materials are another story too. Meteoric iron? Total nonsense unless you're going to say that it's magical. It was only used for actual historical weapons until people figured out how to smelt iron ore. Folding the blade a thousand times for a Japanese katana? Pfft - the Vikings were making pattern welded blades centuries before. (I'll talk about this in another post, probably.)
Having the Right Weapon
See, this is part of the issue I have with the trope - it tends to glorify the sword above other weapons without recognizing that, by and large, swords are not the optimal weapon to have in every situation. Ancient armies had a number of weapons for different purposes, and their choice of weapon was dictated as much by logistics as by tactics. For example: swords were good all-rounders for large, massed melees, but a flanged mace was a better choice for crushing armour. Spears and pikes worked well against cavalry charges. Bills, or billhooks, were perfect for dragging horsemen down, especially heavy cavalry. Rondel daggers were perfect for stabbing through the gaps in plate armour.
This doesn't even mention the popularity of ranged weapons - javelins, slings, bows, oh my! They were all part of the army, and they all had their place and their purpose.
Good vs. Bad Storytelling
The trope of the unbeatable swordmaster is not bad, per se. But it falls down simply because it's over-used. As a method of distinguishing and elevating the protagonist, it tends to get very predictable, very fast - and it leads to predictable plot elements. I think it's better used as a base - a jumping off point, as it were - that can be expanded in other directions. The trope just has this awful tendency to fall into the territory of lone elite; misanthropic, set apart by talent/skill/experience, the constant test against others who practice the same skills, the unwillingness to fight... You get the idea. The concept of the ultimate warrior becomes the whole of the character, instead of one aspect of the character.
Funny thing, though - one of the characters in my book is a swordmaster. I got to thinking about a lot of this simply in the process of character development. Hopefully I've managed not to fall into the trope myself!