I'm with the others, double clawshots would be the best item. Not only are they one of the more useful items among Link's inventory, the sheer mechanics of it amaze me. Even though we do have some form of hookshots now, they're very primitive, non-repeatable, can't pull you towards the objects like the clawshots can, and can't defy gravity by shooting in a straight line. If I had that, think of the possibilities...