What is the hardest puzzle?

the one nobody has solved

The primary purpose of a puzzle is not to be hard but to be interesting and beautiful to humans. To find the hardest puzzle you will therefore have to look for puzzles not created for humans but for chess engines. They exist to test engine capability but not for their entertainment value  But note that even the dumbest individuals have created terrifically hard puzzles by just following SF analysis. Puzzles without attractive ideas and just a long series of analytical moves leading to the quickest mate. No fun whatsoever.

Currently a large number of computer-like challenges is posted on chess.com. They are halfway the human - engine mark. What humans and engines share is a taste for systematic maneuvers - like staircases and king walks. Only humans get bored when the repeats continue for 100+ moves. The hardest puzzles lack even the systematic bits.

You get it, I have no hardest puzzle for you since I am insufficiently interested to find it. I have seen tons of brilliant puzzles but most are relatively easy!