Currently, virtually every MapleStory character profile is scaled off of Empress Cygnus for dropping her floating island of Ereve in the bad future where she was corrupted and turned into a minion of the Black Mage. The end result of the calc clocks in at 4.883-8.01 Petatons, which is Multi-Continent level.
Now, why are they scaled to this?
MapleStory has three beings called Transcendences that govern Light, Life, and Time and are thus responsible for everything in Maple World (the planet the story takes place on, even the stability of its space time. As a result, they are undoubtedly the most powerful characters in the series.
How does that apply here?
During the Cygnus Awakening update, the trauma of the Holy Bird Shinsoo's death caused Empress Cygnus to awaken her latent power as someone along her line was meant to become a Transcendent. However, since there could be only three Transcendences at any time, she only partially transcended, becoming a Half-Transcendent. As a result, she's able to command incredible power herself, but it pales in comparison to an actual Transcendent.
Every character who has received this scaling has either fought a Transcendent (or Transcendences) and survived or battled with someone who had done so, thus placing them at a level above Cygnus battling against such beings.
The Black Mage is the Transcendent of Light and thus one of the most powerful characters in the series. Thus it is only natural that he be superior to a Half-Transcendent like Cygnus, thus he receives the scaling.
In addition, he has other feats that corroborate this scaling. With The Demon's assistance, he managed to defeat and steal the powers of the Transcendence of Time, which bolstered his powers further. He then created the Mirror World (an alternate dimension which is large enough to house a complete replica of Maple World, which is once again a planet) to trap the Transcendence of Time's successor (after splitting the being into two separate, weaker souls and keeping them apart from each other) to prevent them from interfering with his plans. Even after they meet again and become the proper Transcendence of Time, they're still weak and inexperienced enough to be considered an almost non-factor in the Black Mage's plans by the most recent events of the story (along with the powers he retained from their mother).
However, since he has not replicated this feat at any other point (aside from attempting to merge three worlds/space-times together, which he is slowly succeeding at), he gets an "At least High 6-A" rating rather than a flat rating.
Alpha and Beta (known collectively as Zero) are the current Transcendents of Time, succeeding their mother, Rhinne. While they've inherited all of her power, their brief existence has left them inexperienced enough to remain weaker than the Black Mage. Nevertheless, their presence alone was enough to cause the Mirror World to begin to break down after regaining their full power, thus causing space and time in a pocket dimension the size of a planet to fall apart.
However, given the fact that they ended up cherishing the experiences they had in the Mirror World, Alpha and Beta restore it in an instant with ease. Thus they prove themselves to be far above Cygnus once again. Thus they're also rated "At least High 6-A", as they have not replicated a feat of this magnitude since.
The Six Legends (more commonly referred to as The Heroes) are without a doubt the strongest members of the protagonists, forming an entire faction of the Maple Alliance on their own (the other three factions contain entire armies). Four hundred years before the events of the story, they battled and sealed the Black Mage, delaying his tyranny for centuries.
As a result, they also get the scaling since they all survived a fight with a fully-realized Transcendent that towers over Cygnus in power. While they lost the battle, the battle and only managed to seal the Black Mage due to Shade's, Freud's (Evan's predecessor), and Luminous' preparations, but the mere act of doing so is what makes me give them their "At least" rating.
This is further corroborated by the Heroes of Maple plotline, in which The Heroes face off against Damien who was already powerful enough to casually defeat Afrien, who fought alongside the Six Heroes against the Black Mage. Damien was also able to defeat his brother, the aforementioned Demon who helped the Black Mage defeat the Transcendent of Time) and later absorbed the power of the Transcendence of Life after she was weakened and sealed by the Black Mage.
Thus, they also get the "At least High 6-A" rating.
Black Mage Commanders
The Black Mage Commanders are the Black Mage's most powerful minions and thus command large swathes of his army. They also receive the scaling as they all fought the Heroes in the past and lived to tell the tale, proving that they're strong enough to be considered a consistent threat.
The Demon is a particularly special case in that he is the only character shown to be powerful enough to shatter the barriers of not one, but two Transcendences as he weakened the Black Mage's defenses enough for the Heroes to storm his stronghold) Thus he's heads and shoulders above all of them until the advent of Damien and Magnus.
The Nova Heroes are the two playable characters of the Nova, a race of half-dragon humanoids who live in Grandis, a planet in a separate yet concurrent space-time to Maple World.
Kaiser and Angelic Buster get this scaling since they canonically defeat Magnus at the end of their storyline. Magnus was not only able to casually one-shot Guwaru, a fellow Commander, but he also casually deflected attacks from Luminous, and received backup from two Transcendences at once (The Black Mage and Grandis Darmoor, the Transcendence of Life from Grandis). Kaiser and Angelic Buster did so while Magnus was further empowered by Kaiserium, the magic sword handed down to all Kaisers that boosts the wielder's parameters significantly and is one of the most powerful pieces of equipment in the series, which he looted from the previous Kaiser's last stand. Thus the current Kaiser's power would have only increased since then since he reclaimed it at the end of that encounter. To this end, their traitorous friend Velderoth gets this scaling somewhat by taking on Kaiser at Level 100, the point in the game that the characters just begin scratching the surface of their full potential and begin gaining access to their full powers.
There are a few of more characters who get the scaling for various reasons.
Even with his logical reasoning restricted by Gelimer's protocols, Xenon (as shown in the tutorial) is powerful enough to easily destroy a clone of Von Leon, a Black Mage Commander, in seconds. He has only gotten stronger since then, going toe-to-toe with Lotus aboard Black Heaven with the rest of the Resistance. Thus he gets the scaling. By extension, Beryl also gets the scaling for fighting Xenon at his peak.
The Explorers, Cygnus Knights (with the possible exception of the Chief Knights), and regular Resistance members are not quite up to par to the examples above, but still receive the scaling since they are able to help take down powerful Black Mage Commanders and even a Transcendent from another dimension when colluding in large groups. In addition, they all fought Damien's Lieutenants, who are powerful enough to maintain the seal on the Transcendence of Life, Thus they qualify for an at least Large Country level rating for now, but this may be subject to change.
Similar logic applies to Kinesis, an Esper whose powers arose from the aftershock caused by the merging of space-times. He's able to take on the same monsters as the Explorers and casually toss them around.