What is the hardest chess puzzle?

I'm often looking for the hardest chess puzzle, but I can't find the hardest (they're all hard).

If you go to the other chess site you can set the puzzles to +400 of your puzzle rating. On here they'll give you puzzles around 200 above your current puzzle rating.

I prefer chess com but sometimes go to lichess to do some puzzles or watch some gm play. I've done thousands of puzzles on here. I prefer the game analysis on this site more.