Actually, I think the most useless item was removed a long time ago. I was playing InfDev 20100618 and found a strange black shape. It does absolutely nothing and can be crafted with iron. I came across it when making a chest plate. It will be shown in the attachment.
Also, I apologize for bumping this thread.
Ignoring the age of this thread, that is the old texture for a minecart, hardly a useless item (I exclusively use them for transportation between my bases, with around 15,000 blocks of railways in one world; they are better than horses or elytra when traveling between fixed locations because you can AFK while traveling), although back then all you could do was store stuff in them, perhaps even just dirt (as the Wiki says):