The Omnitrix, holding the power to turn anyone who wears into any number of alien forms. With any number of superhuman powers of super-strength to pyrokinetics to even reality warping strong enough to make a copy of the universe.
Variants of this device are the buggy prototype, the finish model (mind the big fat fingers and pressures), the Ultimatrix which allows for super-evolved forms for combat of known aliens, and a version for animals which contain the natural predators of the aliens in the normal form.
Potentially one of the most powerful weapons/tools in the universe it originated in, it becomes more dangerous out of it. For who knows what happens when people with superpowers in their genetics are transformed into an alien. Maybe they only keep their powers in their original form? Maybe the powers transfer between form? Maybe they gain new powers in new forms?
Which makes it interesting when this person was to get the Omnitrix, powers or no powers....