Smooth stone. Literally the most god-awful block for building purposes in the game, I know you can make blast furnaces, armour-stands, slabs etc. with it, however it beyond doubt the most pointless, badly deployed, ugly, and downright awful block in the game.
I am going to take the opposite route and present my point about how tuff is, in fact, a rather pretty block, very useful for building with cobblestone and similar, in gradients, etc, and if you talk to a builder making a diorite to deepslate gradient who doesn't use tuff then they deserve to be burned at the stake and put through Sisyphus' trial.
Smooth basalt, calcite, and amethyst all have rather similar purposes, i.e., none apart from looking rather pretty.
