You can choose any one spell, potion, artifact, ritual, plant, or animal from any of the Harry Potter books. Which would you pick?
If you select a wand, it won’t work since you’re not actually a magician.
Almost every spell would be useful. There are no bad picks. Aguamenti gives you clean, drinkable water whenever you might want some. Accio brings things to you, and would probably get constant use. Reparo repairs things. You could start a business with that spell!
Most of the spells that appear most often are defense-related. I think many people might gravitate to these. This is a mistake propagated widely by the fantasy genre. Harry’s favorite spell seemed to be Expelliarmus, used to disarm an opponent. How often in real life do you need to knock something out of someone else’s hands though? Even police officers might find it to be of less use than they might initially think. If you step up to the killing curse Avada Kedavra, you now have something that is about as effective as a gun, which is only helpful if you actually do intend on killing someone.
In real life, something like Imperio would be very useful. In the Harry Potter books, the Imperius curse is “unforgivable” and only used by bad people, but since none of us lives in magical Britain, taking control of other people seems within bounds. Legilimens is similarly awesome. It lets you read a person’s mind. It’s hard to even begin to say how useful such a thing might be.
Of course, once we’re considering such things, “ethics” becomes a concern. You could certainly make yourself quite rich through legilmency, but it’s hard to see how you might make the world a better place through it. Reparo seems like good, honest work in comparison.
Oh, polyjuice. Is there anything you can’t do? Polyjuice (while a convenient device if you are a book writer) seems like it could end very, very badly if things don’t work out exactly as you’d like them to exactly when you’d like them to. So, no.
Potions are often nice, but trivial. There is a Beautification potion, for example, which could be put to all sorts of nefarious ends. For those who prefer a more direct approach, there are love potions, which both are creepy and miss the point.
The best potion is Felix Felicis. It seems to make all your subsequent choices optimal until its effects wear out. This makes the potion an obvious choice for the right answer that I think it should only be allowed for those of you who have written themselves into the Mariana Trench of plot holes.
I asked the question of a real-life friend who said she’d want a broomstick. This seems like the kind of thing that would get her an awful lot of stares and a visit from DARPA, not to mention the FAA.
There are a large class of artifacts and spells that should be dismissed because they are a bit too obvious when used and might not be looked upon kindly by the nonmagical. You should pick a type of magic that either won’t get you caught or nonthreatening (e.g. Reparo). Similarly, however cool and useful it would be to send messages with Expecto Patronum, I think it ought to be skipped.
The artifact Harry gets the most mileage out of is the invisibility cloak. Such a cloak would be great fun, but I once again struggle to think of what ethical use you would really be able to make out of it. Frankly, I’d find it scary to own such an object because there would be so many opportunities to make unethical use of it.
A lot of the artifacts that readers might be most interested in aren’t plausibly useful. The sorting hat is fun, but really has only one function. Godric Gryffindor’s sword seems awesome, but are you really going to carry it around? Additionally, you’d lose to anyone who draws first with a modern gun. And how often do you need to kill people anyway?
Hermione’s Time Turner has to be among the top three candidates. You could become arbitrarily rich through day trading. If that makes you squeamish, you could simply get more work done in a day. All sorts of mischief would be enabled. Ethical considerations again limit the this artifact.
My personal top choice (after Felix Felicis) isn’t actually used by anyone in the narrative of the books. It’s the Philosopher’s stone, which transforms any metal into pure gold and can be used to produce the Elixir of Life to make the drinker immortal. The Philosopher’s Stone would be the most useful artifact even if it only did one of those two things.
There is the ritual to raise Voldemort from the dead, which probably ought to be skipped. Then again, could this ritual be used to raise others from the dead? Could we use this as a way to get Albert Einstein, Richard Feynman, or Tupac back? Depending on who we could get back in what condition, this might be more important than the Philosopher’s Stone.
Unbreakable vows seem like they would be useful. Certainly if they could be made widely available, they would be used all the time. Since we are talking about what you as an individual should pick though, it might not be of that much use.
Plants and animals
Yes, I know you want a unicorn. But what are they actually good for? And maybe you want a dragon too, but to what end? Where would you put it? Bringing reproducing magical plants or animals into our nonmagical world seems like a Bad Idea.