Hardest Programming Language

Varies. Lots of esoterics will be the 'hardest' just because they often aren't quite turing-complete or documented properly, or just have really odd rulesets that you need to follow.

But I'd pitch in for functional languages, like Elixir, Haskell, Erlang and F#. But that's mainly because functional paradigms are much harder to grasp than OO or procedural paradigms. But once you get it, god-damn, you're a fucking good programmer.