The rules are:
- It can't exist in the real world. (You can't ask for the Hope Diamond, for example.)
- It can't be boring. (Okay, so that's subjective. Sue me.)
- You have to explain why you'd want it.
My son, for example, probably wants a tank that turns into a giant robot. My wife might be interested in having her very own TARDIS. For myself, I'm torn between a Power Ring and Blue Beetle's scarab. (Both of my answers basically amount to Vast Supernatural Powers, only they're item based; both include the possibility of personal space flight, self defense capability, and an enormous amount of personal resources.)
What would you want?