What is the hardest positional puzzle(-s) that you have ever seen ?

What is the hardest positional puzzle that you have found in a book for positional play, on internet, in video, in game of famous players,etc?

I still have no clue solving this puzzle:

lichess.org/editor/rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

In fact it is quite important!

@Sarg0n , For me that's the easiest puzzle and the hardest for me is lichess.org/editor/K7/8/8/8/8/8/8/7k_w_-_-_0_1

@WillBeAGrandMaster said in #3:
> @Sarg0n , For me that's the easiest puzzle and the hardest for me is lichess.org/editor/K7/8/8/8/8/8/8/7k_w_-_-_0_1
I agree Mark Dvoretskiy and Yuri Averbakh have never analysed king endgames . This must be too hard to understand even harder than 2bishops + king vs 1 knight + king

@Sarg0n said in #2:
> I still have no clue solving this puzzle:
>
> lichess.org/editor/rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
>
> In fact it is quite important!

How's that position important to you? Have you switched to no-castling-chess? :p

Castling right are given... KQkq