I would have to say sponge since it doesn't even look good unless your building a giant sponge or coral reef (which still wouldn't look good) I would say the dragon egg or dead bushes but they can be used for decoration in some instances (temple or something to house the dragon egg on a pedestal or something and bushes can be used for various deserty builds).
lapis, sponge, wool colors in survival, dragon egg, cookies, beans, gold tools/armor, gold, dyes and poisonous potato.
And in my opinion there only should be only iron hoes, because IIRC in beta if you walked above farmland it trampled it, now is only by jumping, so no diamond hoes are needed, there should be only iron hoes, a better balance point.
Cauldrons. Now, here's my reasoning: There was a time when cauldrons were useful for brewing stations that were set up in the Nether, since Nether Wart only grew there. But since 1.3, Nether Wart has been able to grow in any dimension. Now what's their point? Aesthetics is really it, sadly. They do look nice, but aren't made to do that. Holding water is a nice thing, and so is collecting rain, but a 2 x 2 of water can easily make infinite water. Plus, bottles can remove water from a single water source block and not use it up. Cauldrons really don't have too much of a purpose anymore, sadly. :c
I think the most useless item is air. It is an actual item, the item id is 0.
The airblock is not usless.. the airblock Id can be used in preset codes, and before you say preset codes are useless.. Preset codes are like forming a world using any block for layers you want, The block i think is useless is Lapiz, it is pretty rare (I havent found "Alot" of it) and it can only be used for dye