Here’s a few simple rules:
The item must be small enough to hold in your hand. YOUR hand, not Colossal Boy’s.
It must be an item that is intended to be used on its own, not as part of a much larger unit: in other word’s, Iron Man’s left gauntlet does not qualify.
Items that can warp reality, create objects from nothingness, replicate themselves, or otherwise make the user nigh-omnipotent are disqualified. No Green Lantern power rings, Cosmic Cubes, or Infinity Gems, in other words.
Whatever liabilities the object has in the story you’re taking it from carry over into the real world. The One Ring is going to eventually corrupt you, in other words, or at the very least reduce you to subsisting on blind cave fish and however many goblins you can murder.
So with those restrictions, what would you want?