Description
Note: this is not a 1v1 tier list, so arguments such as "Well x champion could kill y in a 1v1" are not applicable. Lower tier champions can definitely kill higher tiered ones, the tiering is a measure of how much destructive and defensive capability they have in a 1vWorld scenario.
Suggestions are welcomed
Universal
Immortal Celestials with unlimited/unknown power to affect the entire universe
Catastrophic
Extremely powerful beings that could destroy all of Runeterra, given the opportunity and the incentive
godlike
Behemoths that could singlehandedly wipe out nations
legendary
Ascended warriors, ancient guardians, and terrors of lore, their destructive capability can fell cities in a single day
Superhuman
Those who have far surpassed the level of mortal capability, whether it be through a lifetime of training, fated ancestry, or expertise in magic
lethal
Elite fighters and spellcasters who have proven themselves to be experts in combat, untouchable and/or incredibly deadly on the battlefield
Dangerous
Capable fighters who can take on a few soldiers without issue
Cute
Not dangerous, but can be very helpful/powerful in other ways
Edit: also fiddle cannot ruin the entirety of runtera with incentive. Only a few cities. He is also weaker in the lore than Evelyn
Edit 2: Jax: "Imagine if I had a real weapon."
On the other hand, some champions seem to be placed a bit too low:
Vel'Koz: Vel'Koz is not only the oldest voidborn, he is probably also one of the most powerful. His laser can destroy most things and he has immense knowledge collected over the years. He can also, according to his in-game (in-client?) description disrupt the "very fabric of the world itself".
Veigar: Vaigar is actually a quite powerful mage, being able to defeat bandits with ease, and also raze big buildings to the ground, indicating powerful and destructive magical capabilities.
Soraka: Her magic is so poweful she is actually burning from the inside out. Yes, she may try to help people, but this wasn't a 1v1 tierlist after all.
Also, Kha'zix and Rengar seem to fit more in the lethal category. They are good fighters, yes, but not much else, and unlike most other in that tier, they aren't actually aren't that powerful. Rengar hunts big animals, but that's it.
Other then those minor issues this list is pretty cool and accurate, nice job!
The Darkin indeed were incredibly powerful, being Ascended warriors corrupted by the Void, they would be in the same tier as the other Ascended such as Azir. However, thanks to the Aspect of Twilight, the Darkin have been weakened considerably by binding them to weapons. In Aatrox's lore, he makes it clear that his current form is but a mockery of the height of his splendor. This was why I demoted the three Darkin (Aatrox, Kayn, and Varus) to "Superhuman."
As for Cho'gath, I'm not sure where you read that he is supposed to be planet-sized (maybe the Dark Star skin line? though that is not canon to Runeterra) but let me know if it's from a reputable source. There is very little information about how powerful he is in the official lore from what I have seen. From what I read though, I found that he has diamond-durability hide and is continually feasting and growing. As of yet, however, I doubt he is at a nation-destroying level, as surely he would have done something terrible worthy of a lore update if that were the case. But yes, I concede that a lot that went into his placement was hand-wavy speculation.
And finally for Yone, in his first life he was already a very close match to Yasuo, putting him at high "Lethal" or low "Superhuman" tier. When he was slain, he encountered an Azakana demon, and was able to overcome and tame it, confining it into the mask on his face. Now, having returned to the material plane, he hunts other Azakana with his new demonic sword. I thought that this warranted him being promoted to "Legendary," but I can understand why that might seem a little excessive. As it is unclear whether Yone can even be killed at this point though, I think he sits at low "Legendary" tier.
Again, thanks for the feedback, and please let me know if you have any other questions or see any other strange tier assignments!
Again, great tier list!