Everyone would have it. Everyone would use it everywhere. There would be no more balance. No one would ever use any other sword ever. Lockdown would never be fun again. Everyone's gear would be built around that one particular sword and using it. No one would ever need to craft anything else after crafting that sword. OOO would go bankrupt since no one is crafting anymore. The game's economy would collapse since the only thing anyone would ever get is that sword.

Did I miss anything?

So, in my opinion, there is no true master sword. However, there are those who say it's the Divine Avenger due it's spamtastic charge attack and stats. Aside from that sword, each sword has its own purpose such as:
Voltedge: Elemental/Normal, Static status effect, and medium speed (Good against undead and constructs)
Final Flourish - Piercing, no status effect, fast speed (Good against beasts and fiends)
Leviantian Blade - Normal, Static status effect, medium speed (Decent against all monsters)
Acheron - Shadow/Normal, no status effect, medium speed (Good against jellies and gremlins)

So, there is no true "Master Sword", as Sorou is correct that everyone would use it and spam it.
A neat idea, but unrealistic, would be a sword that does okay damage with every type of damage. Although that sword would lead to confusion when dealing damage to monsters.

*agrees with Ndognine*

1) Suggestions forum exists.
2) This is an MMORPG, not 'single player videogame where there is one weapon to rule them all'.

Even in those kinda games, they're only available AFTER you finish the game, e.g X8 Ultimate armour, Kirby nightmare in dreamland Master sword etc...

If such a thing existed, everyone would have it, and there wouldn't be a point in having it. Everything would just die instantly/way too easily, and there'd be nothing fun left in the game.

Seriously, once you finish everything, have the most powerful stuff, there's nothing much left to do. X8's Ultimate armour features Nova Strike, an attack so powerful it reduces a normal bosses HP to 4 in a single hit, AND it's reusable. Game gets a bit too easy after that. Can beat all the bosses one after the other in 5 mins.

wth is up with that

the master sword is a sword with an epic story/legend accompanied by cutscenes with catchy sound effects and stuff creating a wonderful atmosphere, but the sword isnt better than all other swords in legend of zelda

besides, OP should precisely specify, what zelda game/manga/w.e OP is talking about, because the few popular zelda games, that i remember having played(e.g. OoT biggoron sword, MM i think some fairy sword or deity, ALttP i think tempered sword or something), had swords with higher DPS than the master sword available in each game

as far as i can tell the OP is merely requesting a sword named "master sword" and the thread resembles 1 of the most meager suggestion threads

i got to admit, it would be nice to obtain weapons in a different way(which, in my opinion characterizes the master sword) other than auction/crafting(/or buying overpriced from npc)/brinks, but bleh who cares
cant imbue every single feature into sk
but thumbs up for cutscenes of any kind. i think thats been mentioned by ppl, who like the artwork in SK, as well

A 'Master Sword' that is superior to all other swords?

A 'Master Sword' that has decent power, isn't game-breaking, and has some story to it that is accessed through progressing a plot, yes that would be neat. Like the blade of a legendary Spiral Vanguard or something that is slightly different and perhaps a little bit stronger than the Leviathan Blade (could possibly even require you to have it to obtain this new sword,) yeah, that is something I could agree to. Could being the keyword. I like the concept of a weapon with some backstory to it, though.

MEFAIL.
MY BETHEREN AND I WILL NOW LEAVE.

/walks out alone.

http://wiki.spiralknights.com/Calibur

it's the closest you get to

EXCALIBURRRRRRRR

it's called the Cautery Sword

Silly Psychodestroyer, you don't get the Master Sword in Kirby Nightmare in Dreamland, you get Meta Knight in a mini-game where you have to speed-run the whole game without breaks using an annoyingly finnicky Sword Kirby Clone with a larger hitbox and less versatility. Oh, and less lives.