Two obvious candidates:
Peak DFG.
I was never a user myself - rarely playing mages - but you would always notice the difference between a good or a bad player depending on if they used this ultra-mega lethal active or not.
ZZ’Rot Portal
Riot released ZZ’Rot portal and no one bought it. They continued to make it more and more OP until people finally understood.
By then it had 20% movement speed near turrets, 60 MR, 60 armor, 150% health regen, the voidlings made a lot of damage to minions and turrets + gave you all the gold even if you were in a different lane. Since the voidlings tower dmg scaled with 15% of your total health, they would casually make a 500+ damage to towers in the mid game.
All of this for 2750, and champions like Yorick and Singed could rush it immediately and be unstoppable, especially given that AP champions and tanks really couldn’t destroy the portal (which in itself had the resistances of a tank).
Any other candidates?
On his peak… Its not an item, its an effect but still.
https://leagueoflegends.fandom.com/wiki/Devourer
Devourer…
One of the most if not THE MOST broken thing in all LOL.
There was a point where that effect was everywhere, on every match.
There where like 5 optionsto choose, but never saw them while this effects was at his peak.
Later, the blue one took more effect, but was because they nerfed devourer.