This is just a really quick blog to help explain why the god tiers of the Undertale verse are placed where they are.
Let's start with everyone's favorite ghost child, Chara. As anyone who has played the genocide route will know, Chara and the Player basically go on a killing spree in the Underground; the former by learning from the Player and realizing their "true purpose", and the latter simply because they can. At the end, whether the Player agrees or not, Chara will destroy the world. However, "world" in this case does not refer to the planet, or a single timeline, but instead the game's entire reality. This can be seen by the fact that when Chara performs this feat, the game crashes, and upon booting it back up, there is absolutely nothing (keep in mind Chara only speaks this dialogue to the Player after they wait in the void for ten minutes). How am I so sure Undertale's entire reality was destroyed, you may ask? Simple. Had only the planet been destroyed, Frisk or the Player simply could have reset from their last save point, and the game would not have crashed, as in no circumstances does Frisk's death do this without an outside force pushing you out. Had only a single timeline been destroyed, the game ALSO wouldn't have crashed, as both Omega Flowey and Asriel later destroy timelines while the overall reality of the game itself remains stable.
As for evidence that the game's reality contains multiple timelines (aside from the above), Sans is very explicit when mentioning multiple timelines during your battle with him, and makes it very clear that the problems with them randomly being created and destroyed is due to the constant meddling and resetting done by the Player. He also makes it clear that you're going to keep wiping out timelines until nothing is left. Just as Sans foretold, after killing Flowey, Chara destroys everything, and after the Player sells them their SOUL, goes on to ruin all subsequent timelines, just because they can. As an aside, before anyone tries to say, "Chara only destroyed the monster world" or something ridiculous; not only is that not even a thing (as monsters and humans live on the same planet in the same universe only separated by the barrier), but Undyne also makes it very, very clear that Chara will kill EVERYONE, not just monsters.
Why trust Sans with this? Well, not only is he the only non-Determination user who actually knows about SAVEs, LOADs, and the resets, but he's also implied to be a scientist, have faint memories from other timelines, and has what seems to be a broken time machine in a room he only trusts Frisk to enter when he's 100% sure they're a time traveller. So yeah.
Now as I said earlier, Sans points out that the creation and destruction of multiple timelines is the fault of the Player messing around with their Determination-based time powers. However, before Frisk came into the picture Flowey possessed this power, and also possessed it for a much, much longer period of time. In fact, he reset so many times, he knew literally every possible outcome of every action for every scenario which he could perform in the Underground. The way Sans talks about timelines jumping around, he makes it sound as if quite a few of them are being affected. This is only from the Player's actions. Can you imagine all the potential timelines created by Flowey's resetting? Resetting done for years and years until he was virtually omniscient? While we don't know the exact number of timelines in the verse, due to Flowey's actions, there's likely enough to qualify for a 2-B rating, which is why Chara is rated as "Likely 2-B" for destroying the entirety of the verse in one shot.
Since Chara's rating has been explained, I'll now move on to Omega Flowey. To put it quite simply, he is where he is because he makes it very clear that when he accomplishes his plan to steal the six human SOULS from Asgore, he will become stronger than Chara. As for why Flowey should be trusted on this, well, because he had no reason whatsoever to lie about it. I've seen the argument that this only referred to early game Chara and Flowey didn't factor in how strong Chara would become, but all throughout the game, even up until the very end, Flowey never changes or even questions this view. In fact, he only becomes afraid when he realizes Chara will likely try to kill him before he can take the SOULs. Oh, and speaking of multiple timelines, Omega Flowey creates MULTIPLE SAVEs during your fight with him, which is firsthand proof that more than one timeline can exist simultaneously.
Onwards to Asriel, who is easily the second strongest entity in the game (behind only a certain canine). The reasoning for Asriel's tier should be incredibly obvious to anyone following along, as he is just the complete, vastly superior version of Omega Flowey, who himself was already superior to Chara. However, there's something very important which bumps Asriel to 2-A; his stats.
"Gasp! Stat scaling!?!!1 Azathoth how could u???"
Hold your horses. This isn't the same as stat scaling in something like Pokémon. Undertale has two sets of stats: internal stats and CHECK stats. Internal stats are not made visible within the game itself and are only found by looking in the game's files. These are the stats that influence how much damage a character can do and how much they can take, just like traditional RPG stats. CHECK stats are, as the name implies, the stats seen when using the CHECK option on an enemy. These stats have no affect on the gameplay whatsoever, and exist only to show how strong one character is compared to another. While they are very clearly shown to be non-linear, they do allow us to see which characters are stronger or weaker compared to other monsters. They're also canon to the world, as Chara makes mention of them (it's possible Sans does as well, though I do not remember exactly where if he does). Oh, and if anyone even thinks about throwing out some bullshit like "lol the stats are totally random tho", there was an entire update to the game in which 90% of the edited content was Toby Fox making characters' CHECK stats more coherent with each other and a better reflection of their power.
As for why Asriel having "Infinity/Infinity" is important...well, remember when Chara destroyed reality? Their stats were "only" 99/99 (Note: Yes, these are only their stats with the Real Knife and Locket equipped, but both of these canonically belong to Chara and, as the ending sound effect and animation shows, Chara will always use the Real Knife when they destroy the world, regardless of what the Player chose to equip Frisk with.). Now, lets ignore almost everything I've said in the paragraphs above and pretend Chara only destroyed a single timeline at the end of the genocide route. When they did this, their stats were 99/99, as I've already said. So what does that make someone who is literally infinite in comparison? Yep, 2-A. Oh, and all of this is only taking into account Asriel using a small fraction of his power, which is why full power Asriel is "At least 2-A".Of course, we can't forget Undyne the Undying. While her tier is slightly less certain, she still has the "likely 2-B" for having stats of 99/99; the same as Chara at the very end of the genocide route. As for why she was beaten by a Chara who was still below their maximum potential, that should be incredibly obviously. Undyne is canonically vastly stronger than Chara at that point, but Chara has the advantage of being able to reset as many times as they need to and learn Undyne's exact patterns, as well as the fact that they have the advantage of causing her vastly greater damage than she would normally be taking due to their killing intent and absurdly powerful SOUL. While Undyne the Undying is very likely 2-B, it is important to remember she is also a very, very weak 2-B (comparatively to other things in the tier), as she only really has raw power going for her while being comparatively very slow to other beings in the tier, as well as lacking substantial hax.
Finally, we have Frisk, though the reasoning for their placement should be absurdly obvious. At incredibly high amounts of Determination, they were able to deal small amounts of damage to Omega Flowey before he had been weakened, at all. Their Determination was also part of what gave Chara their power. Finally, at their peak, they were even able to tank hits from full power Asriel and resist him trying to erase them from existence.
There. Hopefully this will stop arguments about the placement of Undertale's strongest characters, for a while. In hindsight it probably would have been useful to make this a while back but whatever.BONUS: You know, just for the hell of it, I might as well explain this little ball of mysterious residue, as well. To put it bluntly, yes, the Annoying Dog is the strongest entity in Undertale, by far.
Don't believe me?
First of all, the dog literally created the game's reality by barking into a text-to-speech device. This obviously includes beings such as Asriel.
Then there's the revelation that you cannot fight the dog due to the fact it's too lazy to patch the fabric of reality, which it is instead sleeping on.
In addition, if an error occurs where the game stops functioning and the world can no longer exist, the dog remains in the void, because again, he was too lazy to patch it. Makes sense, as attempting to go into the game's files and remove the Dog will result in messages telling you that is not possible.
Yes, it is a very strange and meta supreme being, but Undertale is a very strange and meta game. The Dog exists as a representation of Toby Fox, the creator of Undertale, in all his silliness, and is an example of if a verse's supreme being simply used its all powerful nature to goof off, as really, there's nothing better he can do. So he spends his time stealing phones and ancient artifacts while leaving behind infinitely multiplying Dog Residue.
As a set minimum tier, the Dog would be at least High 2-A, as even beings such as Asriel are quite literally fictional to him, and he exists independently of the world they're bound to, since he created it in the first place. However, since he is really an impossible character to judge the exact potential of, due to the fact he exists pretty much only to be funny and cause mischief, he is placed at Unknown.
BONUS 2 - RESURRECTION: Since I'm doing this, it's probably best to cover the "pseudo god tiers" of the verse, as well; characters well above the majority of the cast in power or hax, though not on the level of the true god tiers. I am of course referring to Sans and Mettaton NEO, so I'll quickly (yeah right) cover where they stand in the verse and what their capabilities are.Ah good ol' Sans. One of the best yet most overexposed characters in the entire game. I'd like to rant at the internet for making some of his most genuinely chilling and powerful lines into memes I now see on an almost daily basis, but that's not the purpose of this blog. Back on topic, Sans is a character many people seem to have a hard time grasping the power of. If you were to ask, "Is Sans really weak or really strong?", the short answer would be "Yes". Allow me to explain.
Sans' CHECK info is technically accurate. He is the "easiest" enemy in the sense that he is physically the weakest (seriously, even Monster Kid is stronger than he is), and he does only do 1 damage. However, that 1 damage adds up, as unlike other enemies' attacks, Sans' projectiles aren't a "one and done" type of deal. They have a special property of, in game terms, "ignoring invincibility frames". What this means is, instead of only dealing the initial damage one of his hits would inflict when it lands, that damage will continue, stacking on top of itself until his target breaks contact with the source of the damage. It also means that the durability of his oppenent is far less of a factor than it is for most other characters, as he pretty much always goes with the "death by a thousand cuts" approach, since no matter what, his attacks will inflict low amounts of damage which will very, very quickly add up. Basically, keep touching his projectiles for too long, and you're dead, regardless of what you could usually tank.
Sans' other means of damage is a little something known as "Karmic Retribution" (note: this is a fan name, as the ability is not really given a proper name in-universe and is usually just referred to as "KARMA" or "KR"). This power seems to confuse some people, though it's easiest to just think of it as "soul poison". When Sans hits an opponent, that person's sins quite literally begin to murder them. As it is explained in-universe, their body and soul are being destroyed by the manifestation of their bad karma. Being hit again will cause the process to speed up, killing Sans' foe much faster. As for how "sins" are judged, if Sans' own judgments of the Player as well as Undertale's sense of morality are anything to go by, it is by LOVE and EXP. While these concepts grant actual power in the Undertale-verse, they are certainly not exclusive to it. LOVE is your level of violence, and essentially reflects how willing you are to kill or harm someone else, and how little that person's pain affects you. EXP represent the aptly named "execution points" that are gained when you kill someone else. Basically, the more you've killed and the more willing you are to kill, the more KR tears you apart.
Because of his durability ignoring abilities and essential negation of certain forms of invincibility, Sans is able to hit far outside his own tier (which judging by other physically weak monsters and Frisk at their lowest, should be somewhere in the confines of Wall level). His fight against Chara shows him legitimately killing a vastly superior (understatement of the century, as this is Chara at their closest to their "absolute" without quite being there) enemy. Does this means Sans could take on Chara at their fullest? Hell no. He'd get destroyed. However, the reason isn't because of a difference in power. It's because at their "absolute", Chara is a full-on conceptual entity who requires no body, no SOUL, and no world for them to exist in. When Sans fights Chara, despite all their strength, they are still linked to Frisk's body, which gave Sans a tangible way to actually fight them. The difference isn't how much his opponent could destroy; the difference is fighting something you can hit and dodge vs fighting something omnipresent and formless. This should give you a good idea of what Sans can and cannot take on.
As for Sans' speed, we really have no exact value, though he's almost certainly the fastest non-immeasurable speed character in the entire game. The reason for this is that Chara hitting Sans, under normal circumstances, is literally impossible. No matter how many times they reset, no matter how much they try to memorize Sans' patterns, no matter if he's awake or asleep...they can never hit him. Chara is only ever able to hit Sans when they sucker punch him by actually breaking the rules of the game. That shows an astronomical difference in speed and skill. You can't really scale him to Chara, since that would be the lowball of the century. Pretty much all we do know is he has some form of spatial manipulation as well as teleportation that lets him instantly pop himself and other things in and out of existence. Because of this, his speed is placed at "Unknown".As we move onto our next character, I want to get an all too common misconception out of the way; Mettaton NEO is not weak. In fact, he is easily the third most powerful monster in the entire game (well...monster/robot hybrid), following right behind Undyne the Undying and Asriel (Frisk, Flowey, and Chara are obviously not monsters). This can be gleaned from a couple of things. The first, and most obvious, are his stats. Mettaton NEO has an absolutely colossal attack stat of 90. This is 10 more than Asgore and Toriel, as well as only 9 less than Undyne the Undying and level 20 Chara. Comparatively, his defense is only 9. However, it is important to remember that the defense stat seems to correspond to the attack stat, as something with 0 defense does not die in one hit (Omega Flowey kindly demonstrates this), and seems to act more as a buffer than an absolute value. This shows that while NEO's durability may not be as impressive as his AP, it would still be within the same ballpark.
Now, since stats are not linear, we obviously cannot use these to directly scale him to Undyne the Undying and peak Chara, but we can use them to say that, at the absolute worst, he is an insane amount more powerful than Asgore. At best, he would be somewhere within the Tier 2 range, though on a lower end than Undyne and Chara. While this is an absolutely huge difference, there is something that points strongly to him being that powerful; the fact he was built specifically to be a "human eradication robot". While this may not sound impressive, one has to remember that this would mean NEO would be expected to take on the strongest humans the world has to offer. While Frisk is an anomaly in many ways, it is not unheard of for other humans to reach Tier 2 levels of power. After all, Asriel using the equivalent to the the fullest extent of seven human SOULs' power resulted in multiversal+ strength, and said strength was required to break the barrier, which was created by several exceptionally powerful humans who, unlike regular humans, could actually use magic.
In short, while NEO could be anywhere from high end Tier 7 to low end Tier 2, one thing is certain; he's far from weak. Chara taking him down in one shot is not meant to show that NEO is fragile and powerless, but instead meant to show just how insanely strong Chara has become.
Note: I have noticed multiple people questioning why it took Asriel's power to break the barrier if Chara and Omega Flowey could legitimately destroy timelines, and since it's a simple answer, I figured I'd tack it on. The barrier is not a physical object. It is a spell put in place by very, very powerful humans which was meant to last indefinitely. Destroying the universe does not mean you have broken the spell. This is even directly shown during Asriel's fight alone, as he destroys the barrier after he had already purged the timeline earlier in the fight, but before everything had been restored, showing that the barrier exists independently from everything else.