Ha ha! I knew there would be one, I just knew it! :DUsefulness: Above all else, a Superstar item is useful. If it isn’t useful, then it isn’t anything. You can write up the best bread oven ever imagined, but characters aren’t carting around a bread oven.And even this is not an absolute, as I have a PC in PFS Organized Play who is an accomplished baker (even owns a bakery in Absalom) and has on more than one occasion actually baked things during a mission.
I made a variety of magic items in the last campaign I ran, many of which were both useful and "cool" in terms of ability (though I will make no claims of their being "superstar"), but the one that I swear was my players' favorite was a magic self heating griddle which had largely been thrown in as flavor for a magical stronghold they were exploring.
That doesn't mean that that item in itself is a good idea for a Superstar entry (I don't think it is, which is why I feel free to mention it), but it is always interesting what you may think is a good idea from a game design point of view and what actually gets your players excited and go "neat!"
(Come to think of it, to date my favorite item one of my own characters ever found was an actual magic Swiss army knife. The campaign wasn't so great but the item was cool.)