Neodymium Magnet

Now that is an odd question. How is it possible for an answer if no one knows what you need, want, could use a magnet for? And there are numerous different sizes, colors, shapes, materials of magnets to choose from. Usually one of the most powerful ones are the AlNiCo ones. That's Aluminum/Nickel/Cobalt if I remember correctly. Many substances can be made magnetic also. And in the higher grades of Stainless Steels, non magnetic at all.
If you just want something to hang on a refrigerator door, not much is required. If you want to pick up a car, I suggest an large electro magnet.

And as noted a steel object with sufficient Nickel and Chrome becomes non magnetic which is what is usually used for surgical materials. Not good to have a magnetic scalpal during surgery.
If a coin is produced using such types of high grade steels, a simple magnet will have little to no effect.
Such steels are now used in many construction projects all the time for numerous purposes.