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?
Dorens blade
Heart of gold.
NGL its either dorans shield or sunderer. Dorans shield legit makes it impossible to lose lane and fall dramatically behind - given SOMEWHAT equal skill. And sunderer simply deals more damage than most dmg exclusive items, plus heals for the same amount. Incredible 1v1 item against every kind of target - though this might to a large degree also be because of the champions that build it.
Others that come to mind would be:
Trinity force (pre item rework), steraks, zhonyas, dfg, zz’rot, banner of command, deaths dance, ardent and spear of shojin also had its time…yeah as expected, lots of bruiser items
I don’t know about OP.
But if you want to know Pro stat wise, the worst item ever made
Then I can provide you said data.
It’s HeartSteel.
Only 2 champions bought it since it’s release (Cho Gath and Mundo)
It has less than 70 Games in a 9000+ Sample.
It also has negative winrate.
It is probably is the item that has the least presence ever in league.
Which funny enough Riot then proceded to make a Band after it’s name.
Hullbreaker?
Zzrot, no doubt.
I have played for a decade. Teemo has been my favorite champ since 2014. DFG teemo was cheesy, but zzrot teemo was downright unfair.
However, I will add BotRK. It is, and for much of its history has been, the strongest single item power spike. There have been points where it was not, but it generally has been.
the original sunfire cape. There was a period of time where sunfire capes stacked. so some evil genius thought up building 5 sunfires on eve, and then killing someone by standing near them. (this is back when eve had true invisibility that lasted a full minute)
Hey I used zzrot, I abused the hell out of it. It was a fantastic annoying item for the enemy, disrupted their wave control and helped me push lane while I backed. I wish it was still a thing
current hullbreaker broken as f*.
133% gold efficient stat stick WHITOUT its passive.
whit the passive u get up to 164% gold efficenty in stats.
thats just insane.
besides sup items and ornn upgrades sup items.
thats the best item in the game by fare.
umbral glaive isnt being mentioned nearly enough
Heart of Gold
Idk wtf you’re talking about because ZZ’Rot Portal has to be one of the weakest items in the game. It was mostly used for fun and it was useful but not nearly OP let alone one of the strongest items in the game.
Innervating locket and udyr
Banner of Command is the first thing that comes to mind. Black Cleaver is a second. There was actually a time where champs would literally rush as many Black Cleavers as they could and a lobby would end with like 20 Black Cleavers in inventories.
In hindsight, Philostone was also defo a super broken item.
Release Jak’sho, the item came out and basically became a staple for anything melee, and even had some ranged abusers like swain and gnar