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?
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