The Most Powerful Magic From the Harry Potter Series According to Fans

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

I solemnly swear that I am up to no good!

Of all the mind boggling and awe inspiring magic performed in the World of Harry Potter, [the] Marauders Map stands out as one of the most powerful and useful piece of magic!

As aptly put up by Fred and George, this map shows details about every person inside the castle of Hogwarts, including "where they are, what they are doing, of every minute of every day!" As for its significance in the story:

• It was because of the map Harry gets to know that Peter Pettigrew is alive.
• In Goblet of Fire, Harry notices a person called Barty Crouch roaming inside the castle at unruly hours.
• In the Half Blood Prince, it helps Harry keep an eye on Malfoy's unusual and delirious movements.
• It was Harry's ally during all those night strolls along with Ron and Hermione and helped him manage his mischiefs.

And hey! What's magic without some fun! Right from the spell to get the map activated ("I solemnly swear that I am up to no good") to [the] hilariously composed "request" asking Snape to keep his nose out of other's business, it exemplifies the witty nature of its makers. What makes the map even more impressive is that it was created by a bunch of students of Hogwarts!

Messrs Mooney, Wormtail, Padfoot and Prongs we salute you!

— Quora user Tejas Av

Though many answers are given, I believe one of important powerful [types of] magic is [the] power of house-elves to fulfill their master's command. House-elves have their own brand of wandless magic that only they have access to, as do many other magical creatures. The upper limits of Elf magic are not known, however it is incredibly versatile and allows them to perform very powerful magic.

Kreacher was able to return from Voldemort's cave just because his master ordered to do so. The cave had an extremely powerful dark magical barrier to forbid such from happening, placed there by none other than the greatest dark Lord of all time, Voldemort. However despite this, Kreacher succeeded where even Dumbledore did not. They can apparate from anywhere even Hogwarts.

Dobby could disarm wizards with a flick of fingers, duel any wizard and perform dark magic. He hexed the barrier that leads to Platform Nine and Three Quarters, a very powerful magical barrier, so that Harry Potter and his friends could not enter.

No doubt they had a different and very powerful [type of] magic.

— Quora user Piyush Tiwari

That's an interesting question, which can't have a straightforward answer. The Wizarding World has too many different kinds of magic and too limited a development sadly to make a fair comparison and point [to] the "most powerful" items. Except, perhaps one mentioned below. So I [will] provide a list on my behalf! Most of these are based on items which depict really awesome and apparently advanced magic and hence the correlation to being the most powerful pieces of magic. I have left out one shot spells performed by people because the following are significantly more powerful "applications/implementations" of magic.

• Room of Requirement: There's no doubt about this. Apparently, to me, it's an extension of the Undetectable Extension Charm which Hermione used on her bag and also some form of basic Legilimency to access what a person might be thinking or wants at that instant. I even thought that the Room constructs the room according to what the person wants, actually indicating rudimentary [artificial intelligence] here (science is magic!) and then also "saves" that Room via the Extension Charm to access it later when someone thinks of something similar. This is really amazing.
• Mirror of Erised: It's similar to the Room of Requirement but it doesn't just show what a person wants but their deepest desire. It probably contains some sort of powerful Legilimency, just like the Sorting Hat. The best part is not the Mirror itself but how Dumbledore was able to extend the magic in order to protect the philosopher's stone. Only the person who didn't want to use the Stone could find it and that too in their pocket.
• The Veil: A plain, straightforward, simple looking archway. Yet, it is more like a curtain to [the] souls of people who have passed away. Maybe the "Death Chamber" had that magic over there and someone found a way to hide it using that veil. Or, maybe someone created it for that purpose in the first place.
• The Pensive: Not only does it allow you to extract any particular memory from your mind, but [is] also able to recreate the entire memory and present it visually and then it extends the magic by allowing one or more people to be able to enter that memory and witness it first hand. Talk about having a personal diary!
• Time-Turners: This has to be on the list no matter and is probably the most powerful piece of magic in the Potterverse. The time-turner could take you back in time — as long ago as you wanted to, or had the patience to turn for it. No restrictions. However, due to some significant [mistakes] by those who used it, they restricted that to a five hour window. But even then, the time-turner is capable of something which the entire scientific community or rather something that almost every person has always dreamed of achieving. It's a loop-hole, inconsistency filled plot object, which does annoy me, but you can't deny it's magicness!
• Horcruxes: One object which is able to store a piece of your soul and tether you to the Living World as long as it's unharmed. You can survive literally anything as Voldemort proved to us.

The list could go on, but to me the above are the one of the most powerful pieces of magic in the series. There's more that you could add to it like Lily's Protection, or anything that Voldemort and/or Dumbledore invented basically, the Deathly Hallows which are very obvious. But for now I will keep this relatively short!

— Quora user Sahil Juneja

While Sahil Juneja's answer is great (I think it's an awesome look at some of the most powerful enchanted objects throughout the series!), I don't think it's necessarily the best answer to this question. The questioner asked for the most powerful "piece of magic performed" by anyone in the books, not the most powerful magic that exists in the books, so with that in mind, here's my suggestion as to the most powerful spell any character performs.

First, see Sahil's preface, since I think it's well written and still applies — it is hard to quantifiably say what is the "most powerful" of any magic performed, so I figured I'd list the most powerful spells cast that I can think of for each of the seven books (spoilers FYI).

• The Sorceror's Stone

Quirrell Jinxing Harry's Broom: During the first Quidditch game of the season, Quirinus Quirrell casts a powerful jinx on Harry's broom. In Hagrid's words, "Can't nothing interfere with a broomstick except powerful Dark magic - no kid could do that to a Nimbus Two Thousand. . ."

• The Chamber of Secrets

Lockhart's Memory Charm: Though its effects were perhaps not what was intended, Gilderoy Lockhart casts a memory charm upon himself so powerful that three years later, he still does not recall anything about himself or his life. Help is even apparently beyond the abilities of the best healers in the country (though he is at least starting to regain some of his lovely vain personality).

• The Prizoner of Azkaban

Expeeecto... Patroonuuuum!: Even by adult wizarding standards, the Patronus charm is considered an unusually difficult spell to perform, let alone master and use successfully against a swarm of dementors. Needless to say, Harry's fully-corporeal Patronus at the end of the third book is an undoubtedly powerful piece of magic. Not only does it have full effect from across a large lake ("time and space matter in magic," remember? but it successfully drives off a large swarm of dementors, saving Harry, Hermione, and Sirius' lives (well, souls).

• Goblet of Fire

Wormtail Reviving Voldemort: In the climactic semi-finale of the fourth book, Wormtail (Peter Pettigrew) performs an "old piece of Dark Magic" (Voldemort's words) to return his "Voldy" to a body.

• Order of the Phoenix

I've got two here.

Hermione's Protean Charm: In order for [Dumbledore's Army] to be able to covertly communicate the times of future meetings, Hermione performs a Protean Charm on a set of fake galleons, causing them to all mimic the state of one "master" coin that Harry possesses. In the words of Ravenclaw Terry Boot, "That's N.E.W.T. standard, that is. . ."

Anything During the Voldemort/Dumbledore Duel: I figured I'd throw this one in there as well. During the insanely impressive duel between Voldemort and Dumbledore in the Ministry of Magic atrium, both wizards cast non-verbal spells the likes of which are rarely seen throughout the rest of the series. These guys are seriously good!

• Half Blood Prince

I've got a few for this one too.

Harry's Sectumsempra: During Harry and Draco's "bathroom duel" (snigger), Harry un-knowingly uses some very Dark Magic in an attempt to defend himself. The Sectumsempra spell, which Harry came across in the Half-Blood Prince's copy of Advanced Potion Making, inflicts serious, near-fatal wounds in anyone it is cast upon.

Dumbledore's Highly Intuitive Cave-Magic: While finding their way into the cave where Voldemort has hidden one of his Horcruxes, Dumbledore again shows his powerful knowledge of obscure and subtle magics, by finding first the doorway in and then the chain to draw the boat within the Horcrux cave, both without the use of his wand. Very little of this sort of magic is seen throughout the books.

• Deathly Hallows

Again, I ended up thinking of a few for this one.

Hermione's Undetectable Extension Charm: In preparation for their journey in search of the Horcruxes, Hermione casts this 'Mary-Poppin'-ish charm on her beaded bag, so as to allow the trio to easily travel with everything they need.

Professor McGonagall's Piertotum Locomotor: During the Battle of Hogwarts, Professor Minerva McGonagall casts a rather impressive piece of transfiguration upon the entire castle, causing all the statues, carvings, suits of armor and so forth to come to life and defend their castle ("I've always wanted to use that spell!")

Crabbe's Fiendfyre: Vincent Crabbe, showing a greater than expected level of spellcasting ability (though a true to character level of intelligence) tries to use the Dark, wildly dangerous Fiendfyre curse to smoke Harry, Ron, and Hermione out of the Room of Hidden Things. Needless to say, it does not work out well for him.

Bonus — The Unforgivables

Without a doubt some of the most powerful magic performed throughout the books are the three Unforgivable Curses:

Imperio: Controls people at your will.
Crucio: Inflicts unbearable pain on the person on which it is cast.
Avada Kedavra: The Killing Curse.

Bonus Bonus — Magic Cast Outside of the Books' Timeline

Voldemort Cursing the Defense Against the Dark Arts Job: As noted by the original questioner. This is an unknown and surprisingly powerful piece of magic which prevents anyone, throughout the entirety of time, from being able to even hold the position of Defense Against the Dark Arts teacher for longer than a year.

Death Creating the Hallows?: This one is sorta pushing it. The Deathly Hallows were theoretically created at some point in time by Death him(her?)self, though they just as likely were created by the exceptionally clever Peverell brothers instead. Either way, the spells required to make such objects are undoubtedly very powerful. This somewhat crosses the line into Sahil's answer, as it is partially referring to powerful enchanted objects as well as the spells used to create them.

Dumbledore's Gubraithian Fire: This one is sort of outside the books' timeline, as it takes place within the fifth book, but is never seen from a primary perspective within the story. Dumbledore creates a branch of Everlasting flames as a gift to the Gurg (chief) of the giants, which "isn' somethin' any wizard could do," according to Hagrid.

As a final addition to Sahil's awesome list of powerful enchanted objects, I figured I'd mention the Marauder's Map. It's an undoubtedly clever, powerful, and trickily enchanted magical object, and throughout wizarding history appears to be completely unique!

— Quora user Fateh Khalsa