Well, given that Gorō Masamune was a 13th century Japanese sword smith, who’s name has become semi-synonymous with superlatively crafted blades?
I’m going with the historical figure. There’s some elements about his life that are, probably, hyperbolic, but I’m inclined to believe he did exist, and was an excellent smith. Otherwise his name wouldn’t have kept coming up among the best sword smiths Japan ever produced.
I don’t often say this, but he is literally legendary, as in actual legends have been written, and probably embellished, about him.
Incidentally, the swords he created carry his name, which creates a situation where there are (or were) real Masamune katanas. They have no direct relation to any of the fictional blades that also carry his name. But, it’s not a single weapon. It’s also probably worth stressing, the real blades were simply very well forged Katanas. The only unusual trait I’ve heard attributed to them is they had a singular reflective pattern.
In contrast the Buster Sword, and other comically oversized swords you’ll see in media would be basically unusable. I know Man-At-Arms Reforged made the Berserk Dragon-Slayer Greatsword recently. I want to say they also made a Buster Sword at one point, but I can’t find the video at the moment.
EDIT: I couldn’t find it, because it was before they added the Reforged to their name. The Buster Sword is here.
Also, if you’re curious, they did do Sepheroth’s Masamune, around the same time.