I immediately though of Jax. But this is a loaded question, because there are a lot of champions that are nerfed by the Institute of War for use in the Leauge.

Hecarim. If you read his lore it says no mortal man can kill him, and implies that there's an entire army of soldiers like him from another realm. So if no one in runeterra can kill him, pretty sure he is the most powerful.

Nasus. Nasus is the embodiment of life and death. Nothing could deny his power, as he litterally eats your soul, and as such completely eatting something, draining their power for his own. Not only that but he can wither and age physical beings to their death, and spirit fire can harm ethereal beings. Not to mention what he could do with his ult, and passive/Q

I've got to say Fiddlesticks on this one, He killed a summoner in his lore.

Lee Sin. Was to be the League's greatest summoner, got too cocky and vaporized a town. That wins this contest in my book.

No there is only Jax. He is the only one lore wise to be nerffed by the Institute. All other champs who have their full power constrained are constrained before they arrive in the league. For example Xerath's power is suppressed when he is initially locked away, when he escapes it's still locked away and he joins the league in hopes to remove the suppression.

On the Fields of Justice Jax has the best win ratio and he is the 1st and last person to be nerffed.

Nocture has also killed summoners.

Renekton
Nasus
Xerath
Karthus
Jax
Hecarim
Maybe Trundle

I'm curious, why would you say Trundle? Yes, his regeneration is ridiculous, but it's already dealing with the leprosy curse. One good nuke from most of the champs in this thread would still probably kill him. Aside from that, it doesn't seem like he has any real power. Or am I missing something?

Okay, so assuming we take off all restrictions placed on champions within the league.

ACTUAL GOD TIER
- Renekton
- Nasus
- Brand

VOID TERROR TIER
- Cho'Gath

VALORAN'S BEST FIGHTERS
- Yi
- Jax

SHADOW ISLES DEMIGODS
- Hecarim
- Mordekaiser (implied, though not in his new lore)

These are the champions, who if unrestricted, would be the strongest of the league combatants.

Why not include Kha'Zix? In theory, if Kha'Zix ate every champion weaker than himself, and continued to do so (Only getting stronger and more adapted each time, thus allowing him to consume even more in a vicious cycle), couldn't he potentially be an incredibly huge threat himself?