quicksort, e.g. the haskell[0] example is quite well known. Problem is, it's not real since it doesn't work in place defeating the whole point.
[0] https://qnikst.github.io/posts/2020-10-18-quicksort.html
It really shines if you want a shortcut on a whiteboard interview, though.
Surely you mean mergesort, that's the classic FP sorting example.