An idle curiosity ...
Elements such as H, C, O, and N are exceedingly useful, not only because our lives depend upon them, but also because they - and compounds made from them - have (collectively) very high commercial value.
Ditto for Fe, Ca, Mg, and many other elements.
So what's the least useful stable element*?
It should not be necessary for life - there are quite a lot of elements which are necessary, albeit in only trace amounts (e.g. Cd, I, V) - but should have minimal commercial value (so elements such as Pb, Rb, and Ag don't qualify).
My guess would be one of the rare earth elements (some definitely do not qualify, e.g. Sm), or Sc, Y, Lu, or Hf.
* let's include Bi here, even though, technically, it's not stable