Kg5 of course
The hardest puzzle ever

It's a retro puzzle, isn't it?
Last black move must have been Kg8 etc ... I'm not good at those at all.

So easy! White can just play Ra8#, b8=Q#, b8=R#, Rc8#, e8=Q#, e8=R#, Qe8#, Qd8#, Qdf8#, Qhf8#, f7#, Qdf7#, Qdg7#, Qdh7#, Qhg7#, or Qhh7#.
You can't solve THIS puzzle!