1 or 2.
Obviously. Every other entity appears largely bound to the world of Runeterra. Aurelion Sol created the very star Runeterra needs to exist in a livable state.
1 or 2.
I can't prove it, but I am almost certain that Cho'gath is the immense, all-hungering entity on the other side of the Void Rift Sol felt, and its power was such that even Sol feared it.
3.
Probably. His power is mostly only implied, but he regularly stops cosmic disasters.
4.
This dude is basically Runeterra's version of Dr. Strange, and constantly is on the move preventing world-shattering cataclysms.
5.
The most powerful of the Ascended, Nasus and Renekton barely managed to contain him.
6.
Not sure where each would individually fall, so putting them in one place. Gut instinct tells me Azir is probably the most powerful, but we shall see.
7.
The most powerful undead champion and the most powerful undead in the setting save maybe the Ruined King. He is the only undead denizen of the Shadow Isles who is too powerful to be subsumed and controlled by the Black Mist, so he uses it to his own advantage, walking through it while he spreads misery. This implicitly places him on a similar or greater level of power to the Ruined King.
8.
All right so first of all, it warrants mentioning that, short of Aurelion Sol and maybe Malphite, Vel'koz is the single largest champion in the game. He's enormous, and some splash arts, like the Battlecast one, make him look even bigger. This would make him a physically formidable threat alone. But he also has incredible magical power, able to deconstruct and analyze any living matter with a glance. In terms of raw killing power, he is actually probably stronger than quite a few of the champions above him. He just seems to lack any power beyond destruction, whereas Mordekaiser can take souls or manipulate metal, Azir can reshape the desert into a new Shurima, and etc.
9.
So in older lore this guy basically stopped a battle by stopping time in the entire battlefield. He's also completely immortal and seems to have mastered time.
10.
just barely edges out Lissandra for the last on my list IMO. Lissandra is very powerful, but she seems to actually be relatively frail physically, if he having her eyes clawed out by Udyr and being IIRC injured and escaped by Quinn. Aatrox by contrast is an incredibly powerful warrior who can destroy armies, and can instill a battle fervor in entire armies, making them unstoppable. He also appears to be the cause of Tryndamere being an unkillable berserker, Tryndamere being easily one of the most formidable of the "normal human" champions.
Honorable mention:
Given enough time and prep work, Illaoi would probably easily take spot 5-7. This is because, in the Shadow and Fortune event, by calling onto the power of Nagakabouros, she was able to single-handedly repel the Harrowing and drive the Black Mist back. But this power is not open to her constantly, and required many hours of prayer and preparation.
Side note: Some might wonder why
is not on my list. The reason is simple. Kindred, by definition, has no power. They are death, and do no more than take a life when it is time. They are completely incapable of taking any real important action. They are less a character and more a metaphor for a naturally occurring proces.