There was probably something that wasn't on the poll that I found more useless, but I can't really think of anything at this particular moment in time. Anyway, I said the Giant's Knife, as it really only served as an incentive to get the Biggoron Sword. I'm not sure if it had the same stats, but it was essentially the same design except for the fact that it broke far too often to be useful. If you got hooked to using it, the only option was to get the Biggoron's Sword, who Medigoron himself mentions.
Onto the other items on the poll, I really don't think any of them were entirely useless. Don Gero's Mask allowed you to gain a heart piece and complete a semi-challenging side quest, although I will admit that it was purposeless after that. I don't see how ANYONE could consider the grappling hook useless as you used it at many points in the game, to traverse the landscape and to attack/steal from enemies. As for the Silver Gauntlets, while they were essentially a weaker version of the item that succeded them (Golden Gauntlets), they did not have a limit as to how many times you could use them, like the Giant's Knife. And if you're saying they're useless because of their less-than-impressive purpose, I don't see how the... Strength Bracelet (the band Darunia gives you as a kid) was anymore useless.