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
