Why not just take it a step further, and go full Jean Grey? She can do everything Magneto can do, and then some. She affects things on the atomic level with her psychokinesis, so she can literally split or combine atoms to be whatever she wants them to be. She could make a nuke out of thin air, or stop a nuke from exploding by turning the isotope into lead. No need to manipulate the iron in your blood when she can just manipulate your blood cells directly.
But this also sort of misses the point of the question, I think. The question was about which powers would be most useful. In reality, I don’t have a lot of use for being able to manipulate metals. It may come in handy occasionally… But unless they go full superhero/villain mode, there’s very little practical use for the average person. Unless you’re a welder or work in construction, because being able to warp metal doesn’t have a lot of practical applications for an office drone.
Most generally useful single ability? Probably the Flash's speedforce. Being able to get whatever you want done in a fraction of the time has broad utility. Plus you're well equipped for any supervillain attacks.
In terms of real world applications, prestidigitation is pretty fucking solid. It’s sort of a wizard’s “do anything” spell. Flavor food, clean something, warm up or cool something, make a small trinket, etc… And since it’s a cantrip, you can use it as often as your imagination will allow.
Your lunch got cold? You don’t need a microwave, because in just 6 seconds of finger waggling, your food is now hot. You wish you had some salt to add to it? No need, just waggle your fingers and flavor it however you want. Forgot to do the dishes, and realize you don’t have any clean spoons to eat it with? No need, just waggle your fingers and now your dishes are all clean. Ice cream is starting to melt? Just cool it back down again.
Alternatively, just like it can be used to clean, it can also be used to soil objects. If someone pissed you off, you could literally soil their pants.
Marvel's Legion, aka David Haller, son of Professor Charles Xavier. Omega-level mutant. Psychic powers that go beyond time and space, astral projection, teleportation, reality warping, mind controlling, pyrokinesis, et cetera. If you could keep your head in check (did I mention it also comes with disassociative identity disorder?), taking down Thanos with a completed Infinity Gauntlet would be a mere trifle.
Any character that can teleport (with their clothes intact) and take along the person they’re holding, without the risk of ending up inside something or someone. Traffic, borders, travel time, and sometimes access to restricted places are basically not a problem anymore.
As a concept All for One would be super broken (the ability to take the powers from others is too broken) it was super poorly used in MHA but in a world where everybody has super powers it's a great power to have.
There are several powers that are just restated omnipotence.
Perhaps my favorite is Featherine Aurora from When they Cry. She is the author of the story you are reading. The world exists to please her (and her audience). She cares little about the pains any individual feels as they all serve her greater purpose.
It's 'Just Omnipotence' but it's written in a way for very meta interactions. But yeah, her power to create and change the world is ultimate, she literally can do anything.
I realise it's somewhat of a curse, but Vandal Savage's immortality.
Ideally, some form of time manipulation though. More than simple time travel. Being able to speed up, slow down, have irl scum saving, age/youth-en people/things... And simple time travel too though.
For some plans I have, a WIP supervillain I'm working on would be amazing. The main hero and his rival villain basically are normal people who have another person from another dimension fused in their bodies that allow them to transform into their superhero/villain forms. The best part about it is how when the transformation occurs, it's basically instant and so long as nobody sees it in person, nobody can figure out you're the transformed person. I have so far planned out that people seeing the transformation through the very rare security camera footage on universe would wonder why you suddenly disappeared and in your place is the transformed, so no need to hide from security cameras (even though you probably should just in case).
Would absolutely love to become the villain because he becomes an anthro goat with very long horns that can bend and move them (limited by horn length) to his will, that are pretty sharp. Only downside is how his horns can get caught in walls and other surfaces.
Would be able to do things I could never imagine without ever having to worry about people recognizing me.