Laughing Fish's top 10 most powerful champions (lore)
Honorable mention:
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10:
Why is he powerful? Veiger is the only mortal to master celestial magic. Not only that, but he managed to corrupt it to his will. However, He is delusional and arrogant, and as such makes foolish errors. But make no mistake: The Tiny Master of Evil is a force to be reckoned with!
9:
Why is he powerful? A strange pick, I know. But Heimerdinger is the most intelligent champion in the game. His inventions and discoveries are pulling Runeterra towards greatness. He will be able to figure out how any other champion's powers work, and find some way to stop them. His weakness? He is physically very vulnerable. Many of the champions listed here could simply obliterate him before he had time to work.
8:
Why is he powerful? He is one of the ascended, and is the former emperor of Shurima. He is rebuilding his once great nation, and intends to conquer the earth. But he runs into a similar problem as veiger. Despite his immense power, Azir still finds a way to overestimate himself. He truly considers himself unkillable, and that is always a dangerous thought.
7:
Why is he powerful? The Prophet of the Void has a connection to a being on the other side of the rift. He can create portals to the void, and can summon horrors to do his bidding. Given his way, he would bring monsters into this world that would consume a living matter. But at the end of the day, he is still just a puppet for a much worse creature.
6:
Why is he powerful? He is the time-lord victorious! He can see through time, and can manipulate it to his own ends. Even if you manage to kill him, he can reverse time back to a point when he was still alive. Unfortunately for him, despite his gifts he is bound by time just like everyone else. There are things he simply cannot change. His gift is also very hard to control, and it is difficult for him to remain focused on the current time, as his mind wanders.
5:
Why is he powerful?
He is one of the ascended, and is the ancient guardian of Shurima. He has limited control over time and the life force of those around him, and is able to siphon their lives to strengthen his own. He is immortal, and has gathers thousands of years worth of experience. Despite his massive power, he is vulnerable to other powerful mages, and has lost much of his old will after his brother descended into madness. Nasus, while still immensely powerful, is still just a shadow from a dead empire.
4:
Why is he powerful? The Terror of the Void is one of the only creatures to summon himself into this world. He hijacked the Leagues summoner magic and brought himself into Runeterra. Thinking they had trapped the monster, they put him to work fighting in the league. They came to realize later though that Cho'Gath allowed himself to be captured, and only remains as it gives him an easy source of prey. He is a vile creature who grows in strength and power as he eats. and he eats A LOT. His inability to work well with others (as he has a tendency to eat them) makes it unlikely this creature will ever amount to more than a nightmare though. He is strong, but you can only do so much alone.
3:
Why is he powerful? They say knowledge is power. And when it comes to knowledge, nobody can hold a candle to Vel'Koz. He disintegrates any who he comes across, which allows him to not only get complete knowledge of their anatomy, their strengths, and their weaknesses, but it also allows him to absorb their memories. He is always getting smarter. As he has absorbed entire libraries and armies of men, his strategy and combat skills are immeasurable. His obsession though creates weakness. He spends all day every day gathering knowledge, and not really doing anything with it. He getting smarter, but really getting more powerful.
2:
Why is he powerful? A controversial choice, I am aware. He may be weaker than some others now, but The Glorious Evolution has virtually limitless potential. As we have seen with the Battlecast skin line, Viktor has the ability to consume and convert all living creatures. Every victory adds to his numbers, adds to his abilities, and adds to his power. He can adapt and evolve his armies to defeat any oncoming threat. If Viktor is not killed quickly, the world may lose any chance of doing so. But he has a limit: Energy. His war machines are virtually unstoppable, but they require a very large amount of energy. This is why he lost out to the number one:
1:
Why is he powerful? Karthus holds the key between life and death. He has formed an ever growing army of the souls of the dead. Even if you manage to destroy one of his soldiers, he simply brings their souls back to this world. He is the only known wielder of oblivion, which is the essence of death itself. But that is not all: Karthus is a master manipulator. Despite the horror of his ideals, he manages to convince those to willingly give themselves to death. Every victory for you means nothing while The Deathsinger lives, but every victory for him makes his armies grow larger.
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(Even if Heimer is actually Smarter than Jayce, Rumble and Viktor) (If we consider skins and "Alternative Universes" from the future (Battlecast / Mecha / Fullmetal), the list would be: BC
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(Even if Veigar has the potential to kill them all and be stronger, its just potential)
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(Kogmaw and KhaZix have the potential to reach more power than Chogath: Kogmaw because he is the son of one of the most powerful voidborns, and KhaZix can evolve constantly. VelKoz may be smart and powerfull, but he KNOWS that Reksai and Cho are stronger than him, in fact, Reksai has an "army" of voidmonsters)
also is a force to be reckoned with as it was something killing people in their dreams. but i guess since in the lore hes now bound to real life and forced to participate in league hes prob not much of a threat now.
- As Prince of one of Runeterra's largest city-states he commands a lot of respect and has a lot of authority. He has huge amounts of resources and entire armies to throw at problems, including many other champions - Garen, Lux, Shyvana, and Quinn all answer to him directly and Galio, Xin Zhao, and Fiora would probably obey him without question as well. When you can throw that amount of talent at a problem you can all but guarantee it's going to go away pretty quickly.
- A tactical genius, or so we're told, he's the other major leader of Runeterra. Though he doesn't command the loyalty and devotion of his men, especially his fellow Champions, quite like Jarvan does, he still has access to a pretty big pool of talent in the form of Sion, Darius, Draven, and possibly LeBlanc, and a city-state's resources to throw at any problem he might find. If he can persuade the less-loyal elements of Noxus to side with him he'd be a very frightening enemy - Whoever he's facing best not let their guard down, ever, lest they find themselves face to knife with the likes of Katarina or Talon.
- Very hard to place in the new environment since much of her old lore was retconned and no new lore has been added, but if we count it she'd definitely be up there. Individually she's far more powerful than we've ever seen on the Rift - She's used Intervention on six people simultaneously before (during Maokai's introduction). On top of that, her armor is indestructible and she's immune to nearly every malady that affects normal people. Combined with great charisma, strong leadership skills, the ability to rally people to her cause, and ten thousand years of warfare experience, Kayle would be a formidable adversary for anyone... and that's not counting what she has back home.
and maybe 
finds another gateway to the Void and absorbs its power, he'd actually be the strongest guy around."