I would nominate one spell and one potion.
The Fiendfyre
Think about it — the ability to conjure conscious and malicious un-extinguishable fire that can burn even the most powerful dark magic. And which can most probably be called off only by yourself. We never see this in its true potential because the user is a novice, but I'm sure you should be able to control the fire demon beasts. This is a nuclear bomb though — destroys everything but leaves you defenseless for some seconds while you conjure. You could get avada kedabra'd in this seconds. Still, destroys anything.
And second, but probably more powerful:
Felix Felicis
Luck is such a nebulous and situational concept that enough of this potion becomes a high risk, high reward end-of-all-things magic that makes sure that you don't lose. You don't lose, no matter what it is that you face. You drink this and suddenly the Fiendfyre decides to go away or kill its conjurer. You drink this and maybe that guy who was going to apparate beside you and kill you does something wrong and ends up dying. Harry drinks this and suddenly the tri-wizard cup malfunctions and does not take him to Voldemort. Make enough of this and the battle of Hogwarts goes down in history as the battle of wow-I-did-not-see-that-coming-how-the-hell-did-we-win-in-five-minutes.
You literally force the universe into developing intelligence to bend itself into doing your will and playing in your favor. The whole cosmos changes so that you get an advantage. The gods conspire so that you win. This is deus ex potion. This is op.
— Quora user Alex Kuttel
