Since One for All is not a permanent mode in League of Legends like Summoner’s Rift or ARAM, you should try out the most powerful and exciting champions in this mode before it gets “shelved” by Riot Games.
Here is a list of champions with “unbelievable” power in the One for All mode:
Hecarim

Similar to normal gameplay, Hecarim will build core items like Trinity Force and Youmuu’s Ghostblade. Coupled with the keystone Predator, Hecarim will become a true hunter, securing kills whenever he appears.
Just imagine a Hecarim suddenly charging in like a race car, taking down a target in a single combo in under 2 seconds, and then multiply that by 5 for a full team of Hecarim to grasp the power of this centaur. With such a playstyle, Hecarim will appear, take down his prey in seconds, and then… disappear, waiting for the cooldown of Devastating Charge (E) to return to the hunt, leaving opponents unable to defend or retaliate.
Olaf

In normal mode, Olaf is already a powerful champion for sweeping the enemy. Now, in One for All, Olaf does not need to do that anymore because all 5 members of the opposing team are the same, Olaf just needs to charge in and “Chop Wood” any champion within sight.
Regarding items, Olaf should build at least one damage item with cooldown reduction, which is Black Cleaver, after that, he can build full tank or full damage and still remain powerful as usual. Against the ultimate Ragnarok, strong crowd control skills like Dragon’s Rage, Unstoppable, and Scorched Earth are all meaningless. With 5 Olaf in the lineup, pure tank champions can only “go dark” because of his overwhelming true damage. On the other hand, if facing high damage champions, Olaf can also build tanky, becoming a solid wall that crushes opponents.
Zac

In One for All mode, Zac is simply a champion that “overpowers with flesh” – playing full tank and outlasting every opponent. Although initially Zac may be at a disadvantage due to short range and low damage, things change when Zac gets a few basic items.
Zac’s main strength lies in his healing ability. When facing mages, Zac should build two items: Spirit Visage (to increase healing from Cell Division) and Adaptive Helm to become… immortal. Otherwise, Zac can build armor items like Sunfire Aegis, Thornmail, and Frozen Heart, which are all very strong. Additionally, his extremely strong crowd control skills (QER) always threaten opponents, even from a very long distance. With 5 Zacs, sometimes opponents only need to get hit by one Elastic Slingshot (E) to be knocked out or even killed.
Lee Sin

As one of the strongest champions in One for All mode, Lee Sin requires players to have at least an average skill level to maximize his potential. Specifically, players need to land accurate Sonic Waves (Q) and execute Dragon’s Rage (R) on 3 to 4 enemies.
In the early stages, Lee Sin is practically undefeated at level 3 due to his high damage and ability to shield teammates continuously with Safeguard (W). Particularly, he is also a traditional jungle champion, so there’s no worry about wasting jungle resources. Ultimately, Lee Sin is one of the most beloved champions, so the proficiency rate among players is quite high, significantly boosting your team’s strength.