I would love to have the ability to create any device or object as long as I can imagine what it does and how it looks. Kinda like the Green Lantern's ring, I think I'd be a bit more creative than making fists appear out of nowhere.
Or, not quite as "game-breaking" (I could summon devices to do basically everything, after all), the ability to copy objects down to the last electron and quark. Even if I just use it to assemble an amazing PC by walking through an electronics store.
Oh, yes, both powers will need a limitation: I could not create matter, just rearrange the particles it's made of to shape these objects, so I would need matter of any kind with the same amount of electrons/protons/neutrons as the object to be created as raw material.
Or, as a kind of everyday, subtle supoerpower: an RPG inventory. Still with some limits concerning total weight and storage space, but without having to drag bags along with me, and neatly arranged in a grid all the time. Only visible for me, of course, for everyone else it would look like some kind of pocket, a messenger bag maybe.