the rope gets my vote but also i have to mention the haunted key for its versitility in presentation, including the leadons for key bending tang removal etc.
i'll often start with the key tied to a rope and do the hindu thief routine were a thief cuts the rope then ties it back, mage cuts through the origional not to restore order.
basicly from a £20 outlay (including special key parts

) plus some rope and you get a ten minuet routine ideal for street and table shows and a easy carry around demo trick setup.